In early May of 2005, the first downloadable content was released for Halo 2 on Xbox Live. This consisted of 3 separate packages that together made some significant changes to the game experience: An update for the game, a free map pack, and a premium map pack.


The Autoupdate

Halo 2 had been out for several months, and had developed some problems. The foremost of these was simple cheating. The most common cheat encountered was what came to be known as standbying. As with most multiplayer games, Halo 2 uses a client-server architecture; one of the players in the game (the one on the fastest connection, ideally) was made the host and tasked with coordinating network traffic between the other players. The game tried to make this as automatic and invisible as it could, but it was still possible to discover who was hosting the game. It then became an unfortunately widespread practice for the host to temporarily disrupt their own network connection, forcing all the other players to stare at Halo 2's own version of the blue screen of death (unlike its namesake, this was a temporary condition) and buying a few seconds of unopposed gameplay. Honorable players tried to fight back by using Live's feedback system, but this quickly became a plague seen in more games than not. (The name "standbying" comes from the button on some cable modems labeled standby, which was pressed to perform the cheat.)

And aside from external factors, there were problems within the game itself. Players quickly discovered that it was possible to grab items through solid walls, if one could get close enough. While not exactly cheating, this disrupted the flow of the carefully balanced maps and many players considered it poor sportsmanship. A bug in the game was discovered that allowed players riding in the Warthog to become invisible and teleport around the map. There were numerous other minor quirks and glitches that were often abused, and again the Xbox Live feedback system didn't seem to have any effect.

The autoupdate, released over Xbox Live and made mandatory for all players, resolved all of these problems. The invisible player bug was fixed. Changes were made to the physics system to prevent grabbing items through walls. Some changes which Bungie was unwilling to discuss in detail were made that made standbying a much less effective cheat- it still happens, since it's impossible for the Xbox to control a person's auxiliary hardware, but it doesn't give the players much of an advantage any more, it just annoys everyone in the game.

An unexpected benefit of the autoupdate was that Bungie took the opportunity to address many of the complaints the veteran Halo community had voiced regarding the sequel. The most common complaint was that the weapons had been nerfed and made less reliable- kills were more a matter of luck and the element of surprise than skill at manipulating the controls, and none of the weapons seemed to do as much damage as before. Some of the low-end weapons were quite weak, which was not true in the original game, which led to newly spawned players being essentially defenseless against players who were lucky enough to grab powerful guns before them. The melee attack was noticeably weaker than in the first game, greatly changing the dynamics of close-quarters combat. Grenades wouldn't fly as far and did too little damage to be effective. The autoupdate made grenades powerful enough to be dangerous again, greatly increased melee damage (to the point where a particular weapon with a giant bayonet-type blade will kill in 1 or 2 hits), and changed several other weapons, resulting in a feel that virtually everyone agrees is much improved over the initial release of the game.


The Bonus Pack

The Bonus pack was free for all Xbox Live subscribers, and consisted of 2 maps named Containment and Warlock. Virtually every map in the original game had been designed around 4v4 team games- players were quick to bemoan the absence of small maps designed for FFA carnage or very large maps designed for the game's maximum of 16 players. The Bonus Pack covered both gaps in the lineup.

Containment

Set in the snowy region of the Quarantine Zone near the Library, Containment is a vast canyon with two massive bases, one at each end. Each base is equipped with a whole fleet of vehicles (a Warthog, a Banshee, and a Scorpion), as well as a gate that can be lowered to allow vehicles to be driven in. There are several paths that can be taken across the level, with varying amounts of cover and accessibility to vehicles. Vehicular combat is made more interesting by the presence of mines- large devices which will erupt in purple explosions when shot or collided with.

Warlock

An updated version of the classic Halo map Wizard, Warlock is set in a crumbling Forerunner temple. Like Wizard, four ramped platforms circle a raised area in the center of the map, and four ledges on the side house bases for CTF or Assault games. Unlike Wizard, each quadrant of the map is clearly unique, with different patterns of broken stone and puddles of water, making it far less confusing to navigate.


The Killtacular Pack

Released as premium content, for US$5.99, the Killtacular pack added another 2 maps, named Sanctuary and Turf.

Sanctuary

This beautiful map takes place in a ruined, rock-strewn area somewhere on Delta Halo. Trees and mountains surround a circular area filled with ragged stone and ancient buildings. In the center is a spiral ramp that gives access to catwalks leading around the edges of the map; at each end is an enclosed area. The map is filled with random chunks of rock, giving protection from snipers and making combat almost a game of cat and mouse. Like Foundation, it's possible to have the Warthog appear on this level, although like the other level it ends up used more for goofing around and causing chaos than as part of a strategy since it's not a good driving environment.

Turf

This map takes place in the Earth city seen early on in the single-player campaign- a landmark from one of the levels can be seen in the far distance. It's a cramped, twisted neighborhood with a destroyed warehouse at one end and a massive crashed alien war machine at the other. The highlight of this level is just how much stuff there is on it- around every corner are crates, boxes, garbage bins, bulletproof shields, and other junk that can be tossed around by explosions (which are, of course, extremely common). This makes the level slightly unpredictable as the movable items get, well, moved, and every time you run down a certain corridor it may look different and require a different path to get through (and different maneuvers and cover when fighting).


The trio of updates improved things a great deal, but more problems are cropping up mere months after its appearance. A new glitch has been discovered called "superbouncing", which lets players get to places they aren't supposed to be able to reach- again, it's not exactly cheating, but it's annoying and should not be in the game. A more serious problem is that of hacked maps. Enterprising players have discovered a method of breaking the signing on the downloaded maps, allowing them to be modified without being detectable by Xbox Live and giving the player advantages. Bungie is expected to issue another update soon (as of this writing) to combat this problem.

Sources:
http://www.bungie.net
http://halo.bungie.org
My own experience with the game.