This article is from the Bicycles FAQ, by Mike Iglesias with numerous contributions by others.
Seats are a very personal thing, for obvious reasons. There are several
types of seats:
Seats like the Brooks models. Usually used by hard-core riders. Requires
breaking in before it's really comfortable.
The usual bike seat, sometimes refered to as "anatomic". Has padding
where your "sit bones" (bottom of pelvis) supposed to rest.
Like the padded seats, except they have a gel (e.g. Spenco Gel) in them
for additional padding. Reportedly, the gel can harden and/or shift,
making the seat uncomfortable.
There are several types of seat pads (gel filled, containing an air bladder,
etc) that can be fitted over the seat to make it more comfortable.
If you are experiencing pain in delicate areas (especially you women readers)
you should make sure your seat is adjusted correctly (see the section on
seat adjustment). Women may need a women's type seat, which is wider in
the back (women's sit bones are farther apart than men's). Most pain
can be eliminated by a correctly adjusted seat, using a women's seat,
and riding so your body becomes used to it.