Beijing's subway system is super. For 2 yuan-- that's 30 cents!-- you can zip across the city on clean, mostly new trains. The biggest downside is that it's too popular, so popular that during rush hours you may have to stand in a slowly moving line to get on the train.
Buying single tickets can be complicated. The ticket machines will process your order entirely in English if you choose, but the machines have a secret: they won't take 1 yuan notes. You have to use coins or a 5 yuan note or buy at the counter.
The workaround is to buy a transit card at the counter. The card costs 20 yuan (about $3) and you have to put at least another 30 yuan on the card. They're easy enough to buy, you just pass along a 50 yuan note to the clerk and say "IC Card". (The clerks don't give change, though, so if you hand them a 100 yuan note, you'll get back an IC Card with 70 yuan for rides.) You can then re-charge the cards at the machines if you need to. The IC Cards also give you a discount for bus rides, knocking down the fare from 1 yuan to about half that-- so a bus ride with an IC Card costs about a nickel. And you can even use the IC Cards in taxis if you present the card at the beginning of the trip.
My advice would be to fork over 50 yuan for the IC Card no matter how long you're in Beijing. Seven bucks means you get fifteen subway rides with free transfers and you don't have to sweat individual tickets. If you have rides left over, you can give someone else the card.
On the subway, whether you buy an individual ticket or an IC Card, you have to use your ticket again at the exit gate on your way out the station, so keep it handy. At present, there's no extra charge for distance, but the mechanisms for doing so are already in place.