Disc Golf is becoming an increasingly popular sport; if no one has invited you to throw a round yet, it's probably only a matter of time. Fortunately it is much easier to learn then regular golf, but the way a disc flies can be every bit as mysterious (if not more so) than a golf ball. The information here is most applicable to Disc Golf, but the discussion of disc flight behaviour applies to all discs. Please note that I assume the reader is right-handed for all my descriptions, if you are left-handed, simply reverse all the directions.

For Beginners

Going out with experienced disc golfers can be very discouraging for the novice. There is a definite elitism that translates into bad advice for beginners. I've repeatedly seen many players giving advice that is way over the heads of someone who simply lacks the muscle knowledge to make precise adjustments to their technique. Here is how you can jump right in and start developing skills quickly:

Consciously release the disc horizontal to the ground.
The most common problems beginners have is tilting the disc up as they release it, causing it to fly very high in the air then dive sharply to the left. Focus intently on releasing the disc level with the ground. The disc will generate its own lift, so you want to throw as straight ahead as possible. There is a time and a place to throw higher, and that will become apparent once you really learn to huck it.
Concentrate first on accurate release before trying to throw as hard as you can.
While throwing hard is a necessary skill, you need to first learn to control where you are throwing it. Many repetitions of slower throws will afford you more control as you increase your power, much like Tai Chi.
Work on spinning the disc fast with a smooth release (ie. not wobbly).
The sheer forward momentum of your disc is only half of the distance equation. Ther other half is how fast it is spinning and how smoothly. Without consistent smoothness, it will be very hard to judge what angle you should release the disc at.

Disc Flight Characteristics

When a disc is thrown, there are many variables that determine its behaviour. Among them are tilt (front-back rotation), camber/bank (side-to-side rotation), smoothness (how much wobble you impart to the disc), rotational speed, and forward speed.
General Flight Model
Without getting scientific, a disc will fly in a straight line as long as it is totally horizontal, has forward momentum, and is spinning sufficiently fast. How fast it needs to be spinning and how long it will maintain its forward momentum depends on the design of the disc, but all the following principles apply to all discs whether they be drivers, mid-range, putters, or even generic frisbees.
Tilt
Throwing a disc up will cause it to fly up (of course), but more importantly, it will allow gravity to fight against its forward momentum and thus rob you of distance unless you have an incredible amount of power. Even the most powerful throwers need very little tilt in most situations. Throwing down can give you added distance since gravity helps you in this case, but of course it only works if you are throwing downhill or it will hit the ground too soon.
Camber/Bank
Throwing with camber to the left or right will cause the disc to turn towards its low edge. It will also allow gravity to pull the disc down, so it can really suck a lot of distance off. The camber of the disc changes significantly as it flies, however, so calculating what your angle of release should be is not always a straight-forward task.
Rotational Speed
Rotation is what holds a disc level (like a gyroscope). A sufficiently fast, smooth rotation will allow a disc to hold its camber indefinitely. Thus, the faster you can spin a disc, the more control you will have over your flight path. When a disc starts losing rotation, it banks to the left (assuming right-handed backhand throw, ie clockwise rotation) and falls to the ground.
Smoothness
Okay, here's the interesting part. If you release a disc and it's wobbling, it will bank to the right (same assumption as above) until its rotation smooths out at which point it will either straighten out or immediately start curving left (if it is not spinning fast anymore by that point). While no release is perfect, poor releases will sap a lot of energy off your disc.
Forward Speed
Basically just propels the disc forward. Without rotation it will not go far, but all the rotation in the world would do nothing to hold the disc in the air without forward motion to create the air flow necessary to generate lift.

Special Considerations

Disc Damage
Ever hear people talking about a disc needing to be broken-in or being too old? Damage to a disc effectively amounts to permanent wobbliness. When you release a disc unsmoothly, its rotation and speed will cause it to eventually stabilize and spin smoothly. If a disc is beat-up, it can't stabilize, and thus will turn right for longer. A brand new disc, on the other hand, may have a tendency to not turn right at all, and may not fly well for you until it has a few dings.
Overstable, Understable
The stability of a disc generally refers to its tendency to turn left. Overstable discs need more spin to fly straight (or to turn right for that matter), while understable discs fly straight longer (and turn over easier).
Driver, Mid-range, Putter
Generally I think this is defined simply by the aerodynamics of the disk. Drivers have sharper, more aerodynamic edges, mid-range have broader edges, and putters have blunt edges. With putters there are special considerations in design, such as making the disc less likely to bounce out of the basket and roll away.
Wind
The way I like to think about it is that a head wind will make the disc behave as if it's flying faster (since more air is passing over it), and so discs will turn over faster as well as slow down faster. Tailwinds do the opposite (which is lovely). Crosswinds are a bit more complicated, they can do crazy things. But the critical thing to remember, is that as soon as a disc banks, a crosswind will either push it up or down, so it is best to try to throw your straightest flyer under these conditions.
Alternate Throwing Styles
Maybe I will expand this section as I gain experience, but there are generally 3 alternate throwing styles that I use.
Sidearm/Forearm: A sidearm throw simply makes the disc spin the opposite way (although the difference in technique will likely result in other differences as well), which is useful if you want it to cut right at the end instead of left for instance.
Tomahawk/Hammer: By holding the disc vertically, an entirely new range of flight characteristics emerges. Hammer throws are useful for getting over obstacles in front of you, and can also be easier to aim in many cases. These definitely deserve their own writeup since the verticality of the disc pretty much negates 80% of what I've already said.
Roller: By throwing the disc with right bank so it hits the ground on its edge and rolls, it is possible to pass under low obstacles and sometimes achieve incredible distances.

Notice that I've talked mostly about how discs behave, and not much about how to actually throw them. That is because people have different styles, and what you really need to improve your throwing is just a lot of practice, and the knowledge of what you are trying to achieve. For those playing Ultimate or DDC this information may also be useful although throwing those types of discs requires much more rotational speed and smoothness. Happy Hucking!