First off, a bow, much more so than a gun, is not "one size fits all". You need to go to a pro to measure your draw length which also determines the shaft length of the arrows you will need. If the bow dosen't fit YOU you'll never shoot it well.
Bow technology has advanced quite a bit in a very few years so there are bargains on older bows out there, depending on what you want to put in one. The name of the game in "break-through improvements" each year is to convince those who bought last year's model that theirs is now obsolete or at least not quite as fast, as light, as accurate, yada, yada, yada / . . . . truth is, most bows made in the last 5 years or so are pretty good. Older than that . . . yeah, can be less than satisfying for a lot of applications.
Cams are are round or elliptical. Present thinking is the best compromise for accuracy and speed is a hybrid with one round wheel and one elliptical cam. Two round wheels are slower but "more forgiving" in shooting, and will be found on longer bows. Double elliptical cams will be smaller and faster but more difficult to tune and keep in top shooting condition.
When buying a used bow, after addressing the above concerns, make sure the limbs don't have any splits, cracks or bad dings that could cause a failure under draw pressure; make sure all the tapped holes are vital and not stripped out; and it shouldn't sound like a maraca when you shake it. Strings can and should be replaced regularly and should exhibit no fraying or trouble spots. All the cables and pulleys should be inspected and found in good condition, as well. It all works together as a pulley system and any one part that is off can derail the entire train.
After that, it's kind of like a 1911 or an AR rifle in that there are thousands and thousands of options in add-ons and aftermarket doo-dads you can hang on one verses what you actually can use or need.
Like buying a gun where you don't want to overpay until you learn if it's what you really want to sink your money in, the services of a knowledgeable friend or acquaintance can be invaluable. It's hit or miss when dealing with salesman just like anything else - some will tell you and sell you anything and claim to be experts when some don't know much more than you do about it.
As far as brands, Browning, Matthews, and a slew of others are offering quality bows. Bear is the old standard but is playing catch-up in the market these days.
Like anything you'll get a lot of diverse opinions and the subject could be covered (and is) by entire books. Hope I didn't muddy the water too much.
