Top Gear is a tongue-firmly-in-cheek show that is ostensibly about testing automobiles on television. Really what it's about (as the hosts cheerfully and frequently admit on air) is giving three idiots a rather large budget to play about with cars. In addition to budget, it gives them the clout of the
BBC and fairly wide television exposure. This is sometimes enough to convince the makers of some of the most insane metal/carbon-fiber/plastic bits of fast on the planet to hand it over for examination, slavering TV-camera-car-porn-shoots, and of course if it's truly outlandish enough the grubby paws of Clarkson and The Stig.
As of 2006, Top Gear is in its eighth season. The show's format has settled down to the point where any individual episode includes (at least) several of the following types of segments.
Car Porn
In these bits, a really choice bit of fast is shown off to the viewers. This usually entails a thorough going-over with the cameras, along with commentary from one of the three hosts - the aforementioned Jeremy Clarkson (The Idiot), Richard Hammond (The Hamster), or James May (Captain Slow). Usually, if it's a performance car, it's Clarkson. The auto is given the benefit of high-performance cameras, occasionally some really choice location shoots (national park scenery, overseas location if it's appropriate, etc.) and we're told a bit about it in voiceover and in walkaround. Finally, the host will drive the car for us, narrating to an in-car camera all manner of facts we'd like to know. If this is a performance car, this will usually also mean taking it on a lap of the Top Gear track, which is constructed on an airfield. Once the host (Jeremy, usually) has flogged the car around the track, we need to find out what the car can really do when not in the hands of "an idiot" (according to Jeremy) so it is handed over the The Stig, their "tame racing driver" ("some say his genitals are on upside down!") to take it on a proper lap. Afterwards, the car's time is registered for posterity amongst its brethren on the Top Gear circuit board for comparison.
The Challenge
These segments make for much of the show's hilarity. The hosts are given a challenge (samples from season eight: "Make a minivan convertible. Now take it up to 100mph, through a safari park, and a carwash." "Make an amphibious car.") Given their own imagination and a budget, they attempt to carry out said task for the cameras, usually resulting in hilarity (I will spoil only one thing by noting that their attempt to take their handbuilt minivan convertible top through an automated carwash resulted in said carwash being set on fire.) These don't always involve actual cars - "host a drive-time radio show" was one such. That didn't go so well, to much hilarity also.
Car Versus
That the car is a better form of transport than (insert method here) is often asserted on Top Gear. How much better? Well for that, you need a race. There have been at least four of these that I'm aware of, and they always involve Clarkson in an automobile attempting to reach a destination somewhere else in Europe ahead of May and Hammond, who are limited to non-car transport. One of the most famous ones was combined with Car Porn - the Bugatti Veyron is raced against a Cessna 182 light plane from northern Italy to London. Clarkson drives the Veyron, and Hammond is flown by May (who is a private pilot) to the North of France, where they are forced to switch to a train because May isn't trained for night flying. Another pits Clarkson in a Ferrari 612 Scaglietti versus the other two in airplanes and trains from the Top Gear studio/track in Guildford to Monte Carlo (I believe). As a rule, the car will win, and you can decide for yourself how stacked the race was or wasn't behind the scenes.
The Contest
One or more hosts participates in a contest against a third party, so long as it involves automobiles. Richard Hammond in a mad Icelandic 4x4 trying to outrun a jet-powered kayak (I kid you not) was awesome. Jeremy trying to get a 4x4 up a fairly famous bog (which, it turned out, was protected wilderness in which he got stuck several times, resulting in problems). One famous contest had Clarkson pitting a Lotus Exige against an AH-64 Apache helicopter gunship - the rules were that both vehicles had to stay within the confines of the airfield test track, and the helicopter had to get radar lock on the Exige. This was much more even than it sounds; the Exige is one of the most nimble cars made, and is (in addition) made mostly of plastic.
The News
All three hosts produce and recount, for the crowd gathered in studio, various car-related bits of news or factoids, recently with LCD monitor and prop support. Decent talk-show comedy with much back-and-forth insulting going on here as well.
Star in a Reasonably-Priced Car
Top Gear maintains a 'reasonably priced car' - until this season, it was a Suzuki Liana, and is now a Chevy of some sort - and they encourage celebrities of all sorts to come in to the show and take it on a lap around the test track. Their current rules are that each driver gets five practice laps, and a sixth is timed, no matter how it turns out. If the track is wet, their time is recorded with that notation. Times are recorded on another 'scoreboard' that is maintained in the studio. Each celeb is interviewed after performing their run, during which time footage of the run is shown and their time revealed. Note: Hazelnut informs me that the Reasonably Priced Car is now a Chevrolet Lacetti which is in fact a rebadged Daewoo, proving that there is nothing I can try to node which E2 does not demonstrably know more about. Thanks!
Car Review
Sometimes even Top Gear admits that they should show and talk about motors which cost less than the GDP of an English county, and these segments are for that. Actual cars that we might buy are demonstrated, and their pros and cons aired for us to consider. This is done with the usual lack of concern for manufacturer or designer's sensibilities; during a recent review of a hot pink Nissan Micra convertible, Richard performed the entire demo drive wearing a bag over his head and was so negative about the car he eventually 'abandoned it' on the road in disgust.
There are other bits which don't fit this broad generalization, but these should suffice to give a flavor of the types of segments the show produces.
Why would one watch this show? Well, really, if you're a car aficionado and enjoy an irreverent approach to the whole thing. Each of the three hosts has a different segment of the car world they enjoy. Clarkson is of course in it purely for performance. Hammond enjoys hobby as well as recreational cars - off-road, convertibles, and classic cars. May is a champion of the sensible motor - something which just does its job and does it well, although he does favor completely non-sensible cars for fun - it was with great joy I watched him review a Triumph TR6. Of course, for maximum silly, there is a great segment where each of them produces their favorite car and hands it over to another of the trio for reviewing. Just watching Jeremy fold his six-foot-plus-much frame into a Morgan V8 and try to push it hard on the track on a wet day while bitching about wooden cars and Hammond's taste in machinery is funny ("Honnnnnestly, Richard..." while spinning it out). Watching May try to drive the batshit-insane Noble M12 and the resulting look of terror is even better. Poor Hammond is left with the 'eminent sensibility' of the high-end Vauxhall (I think) that May showed up in, and his boredom gives full range to his sarcastic nature. Recently, I've been having trouble where his sarcasm and appearance lead me to confuse Hammond in his more ADHD-like moments with David Tennant's Tenth Doctor, and I keep waiting for him to whip out a Sonic Screwdriver and dive underneath the dashboard of some recalcitrant car.
The Stig is a deliberate enigma. Never speaks a word, and is never seen out of his racing gear, helmet closed. There have actually been two Stigs; the original 'black' Stig (so named because his helmet and gear were black) and the current 'white' Stig (for obvious reasons). The black Stig 'died' at the beginning of season three while attempting to match the takeoff speed of an AV-8B Harrier jumpjet aboard the HMS Invincible - using a raggedy Jaguar XJS fitted with nitrous oxide injectors. The car flew over the 'ski jump' flight ramp 'due to inadequate braking' and all that was found of the black Stig was a driving glove. The replacement white Stig once listened, as a prior writeup said, to power pop while driving; in season eight, he has switched to instructional 'Learn Italian' CDs, and we are treated to sing-song examples of the proper pronunciation of gradirei un gelato as he screams into a corner during the testing of the Koenigsegg CCX. Note: by season eleven, the Stig has moved on from 'Learn Italian' CDs to listening to raw Morse code, the deciphering of which is left to the rabid TG fanbase with much hilarity. (thanks to auraseer for pointing this out!)
The show is vastly entertaining. It can be found (I believe, as I am not native) on BBC2. There is a website at http://www.topgear.com which has full details, as well as HTML versions of their reviews and a hilarious collection of 'found photos' of truly hideously modded cars titled 'carbage.' Torrents of each show can usually be found in the usual places around them there internetz the day after airing. In the benighted Colonies, it can be found (one or two seasons behind) in the evenings on BBC America.