When playing ring (or "cash") games, there is almost always the choice between a "full game" (usually 9 or 10 players) and a "shorthanded game". Any time a poker table has less than 7 players sitting down, the game is usually said to be shorthanded, but you will most often be able to specifically choose a table where no more than six players can sit down, referred to as "six-max". This may be different for different sites, and there is a risk that the specific limits and type of games you play are not offered on six-max tables. A full game is usually seen as the norm; keep that in mind if you ask a strategy question - the person you ask will often presume that you have at least 8 other opponents unless you tell him otherwise.
In a full game, it stands to reason that you will not win as many pots as at a smaller table - you will only hold the best hand an average of 10%, as opposed to maybe 15% of the time - but because there are more people involved, these pots can still get big. You will also get to see 10 hands for the price of one big blind and small blind, as opposed to only 6 hands at a 6-max table. This makes it ;correct for you to play more tightly; it gives you the opportunity to wait for premium hands. Be warned, however, that your opponents may well be of the same mindset. The hands your (non-maniac) opponents hold are often strong as well. Playing in a full game will teach you the invaluable lesson of patience, and it's important that you learn it.
Shorthanded is a little different, however. Because there are fewer players at the table, the relative strength of your hand will be greater on average - because after all, you don't need a strong hand to win, just stronger than everyone else. Many of your opponents will take this concept too far and simply go hyperaggressive with virtually any hand, so unless you're willing to put up a fight when you have something worth playing, they will take your money away simply by betting and raising. Waiting out the same premium hands as you might in a full game simply will not be enough to maximize profits at a shorthanded table. You may still win or at least break even, but you're missing out a lot of potential profit if you're often folding the better hand. Observant opponents will notice this tendency of yours as well, of course, and simply not pay you off when you do get a strong hand. Then again, worrying about observant opponents is not something you will have to spend a lot of time doing at the very lowest limits.
Because shorthanded games to a larger degree reward looser play, it's also the home of the action junkies, making the pots big and the swings even bigger. A word of warning - if you intend to play shorthanded tables, do so properly bankrolled. The swings are huge, and can break you if you can’t handle them. 300 BB should be enough, but I would still tread carefully unless you're very skilled. If you currently play limit full ring and want to try shorthanded, step down one notch in limits for the shorthanded games.
With that said, there are also very good reasons to play shorthanded, not the least of which is that they can be some of the most profitable games found online. But playing shorthanded is also a great learning experience, specifically because you will start to play more marginal hands and will find yourself in difficult decisions very often. If you make the most of it, you will learn a lot from these situations, and this experience will be noticeable in any other game you may find yourself in, too.
Finally, a word on playing heads-up in a cash game: Don't. Unless you're playing the absolutely worst donk in the world and you happen to be an expert in playing specifically donks, there's almost no skill gap in the world that can overcome the burden of rake in a heads-up cash table. If the table you're playing is starting to get shorthanded, and more people are leaving, you should go, too. Don't be the guy who sticks around playing heads-up just because you're down some money. Find a better table. I know just how tempting it is to stay and win your money back from the guy who outdrew three times in 12 hands, but just the fact that you think that shows that you're on tilt. He is not your goal: Winning money is your goal. And playing heads-up here will only hurt your bankroll. If you really must, at least try to challenge him to playing a heads-up SnG against you. That way, the rake won't make you both losers.