OK, You go to your friendly neighborhood music shop and innocently ask for a drum head. Geez! Next thing you know the salesman wants to know sizes, double or single ply, coated or uncoated, brand, resonant or batter, hydraulic, built in e-rings or not, and a host of other things designed to make you feel like an idiot. You attempt to slink from the store, and vow to try and get a fourth year of use out of your current set.

Well, maybe it isn't quite that bad, but with the current variety of heads on the market - your choice is startling and can be confusing. Lets try and sort some order from the chaos.

The Importance of Heads
The drum head is where the sound of the drum starts, and is vitally important to the sound the drum produces. Approximately 85% of the sound produced by a drum is the heads. A good set of heads can make a cheap kit sound good - while a cheap set of heads can make a solid maple, top-of-the-line kit sound bad. Old heads which are stretched or dented cannot produce a good sound.

Sizes
Any standard kit made in the 70's through to the present day is most likely to consist of a 22" bass drum, 13" - 14" - 16" tom-toms and a 14" snare drum. A six piece kit will have an additional 10" tom-tom, while a seven will include both a 10" and a 14" tom-tom. Kits made in the '60s will most likely have the same with a 20" bass drum, while 50's and earlier kits may have 24", 26" or 28" bass drums. When in doubt, measure. Or just take your drums through to the store.

Double, Single Ply & Hydraulic Heads
A single ply head - is when there is only one layer of material (usually Mylar plastic). These heads have more of a "crack" (sharper attack) and "ring" (sustain), but are not as durable. Many consider these the better sounding heads.

Double ply heads - Have two layers of Mylar, are thicker and more durable, "dryer" (less sustain), "thicker" sounding (less attack and overtones). Definately the choice for harder players, where durability is paramount.

Hydraulic Heads - are double-ply heads with a thin layer of oil between the two layers. Very "dry" sounding and very durable. Often the choice for very hard hitters or those who want a very dry sound without damping. The only heads I know which are hydraulic are the Evans Hydraulic series. Remo Pinstripes, which are often mistaken for hydraulic heads, are not - a clear double-ply head has an oily rainbow affect as light refracts differently off the two layers.

Resonant or Batter
Simply put: The Batter head is the top head, which is struck - this may be single or two-ply. The resonant is the bottom head - which should always be single-ply. Although any single ply head can be used for a resonant head, some makes now have specially designed resonant-only heads (usually quite thin).

Coated or Uncoated
A coated head is a batter head which has a rough white texture on the top surface. This has the effect of killing some overtones - giving a drier sound. Most snare drum batter heads are coated, and if you use brushes, this is the only type of head you should use.

E-rings
Some newer snare and bass drum batter heads have an extra thin ring of Mylar under the head. This has the same effect as using an E-Ring (donut) - damping overtones, and giving a focussed "crack" without the need for extra damping. Unlike using an E-Ring on the snare, a built-in ring will not affect the sound of a side stick (where the tip of the stick is held against the surface of the head, while the butt is hit against the rim of the drum). The advantage of using the bass drum heads is that not needing additional damping (pillows etc.), the drum is much louder.

Brands
Whatever you prefer - try different brands to see (I went through 15 makes/models in my first year of drumming). In my experience Remo generally have a thicker sound with more overtones, while Evans - due to the CAD/Cam manufacturing process - are more consistent. If you have a more note oriented specific tuning system (a-la Neil Pert) the latter would probrably suit better.

How often should they be changed?
This varies depending on tension and playing style. One sure sign is dents - the moment dents appear your heads are shot, although an undented head may be stretched and unable to produce a clean note. On average:

Snare drum batter - Monthly (due to it being the highest tension & most often used)
Snare drum snare side - Every 3 Months (High Tension, very thin head)
Bass drum batter - 3-6 months
Bass drum resonant - 1-2 years
Tom-tom batter - 3-6 months
Tom tom resonant - Once a year