This is probably the easiest thing to figure. You probably know the answer already, and if so, you can move to the head of the class: VHS or Beta? More than likely, since it became the industry standard, the format you'll require will be VHS. But what about this thing, S-VHS? What's up with that?
S-VHS simply describes a more advanced wrinkle to the original VHS, which came out in 1976. In short, S-VHS offers more lines of resolution than regular VHS, which results in a better picture. Also S-VHS players feature backward compatability. So if you're shopping for a player, keep in mind that a VCR that can play S-VHS can also play regular VHS.
What about Beta? This was a tape format offered by Sony, which won the battle, but lost the war. In other words, when it came out, most folks regarded Beta as superior in quality, but it didn't have the market share of VHS, and subsequently couldn't compete. If you have stuff shot on Beta, why then, you would need to look for a Beta playing VCR. But then, you probably knew that already.
With a lot of ads, you'll see the back of the unit, which is helpful because it gives you an idea of how to connect the VCR to whatever you need to connect to. For example, you'll see a video connection, usually yellow, and audio connections, usually red and white. Sometimes you'll see an S-video connection, which is sort of an all-in-one connection, and the best connection. It makes sense to work backwards from the back of your TV to figure out how you're going to connect the VCR and the TV together. Oh, yeah, don't forget to deal with the cable box or antenna into the setup. Think: cable box/antenna > VCR > TV.