I watch basketball, I play basketball, I know basketball reasonably well. But one thing about basketball has never made any sense to me: the positions. I understand them in general - the point guard brings the ball up the court, the center and power forward play in the paint and generally post up, etc. I get that the point guard will run the screen roll and the forwards will set the screens and such, but what is the difference between a shooting guard and a small forward? And sometimes they will say so-and-so is playing out of position, that he is playing power forward when he should be playing small forward or guard. When I play pickup, nobody worries about positions, we just run and try an occasional play, so clearly positions can’t be THAT important. What gives?
When you play pickup games you aren’t running plays, for one thing. A team that’s running a particular offense and a particular defense for an entire season is going to require every player to know where to be, and it’s going to call on each player to fulfill a certain role. If your 3 man is running the baseline mostly and shooting open 18-footers, but you have a power forward playing “out of position” at the 3, even though he’s a six-eleven shotblocker who can only really play around the rim, position matters.
You also aren’t really worried about matchups in a pickup game. It’s all relative, after all – you only need a center to the extent that you define a role for the center and use him a particular way, so when everybody’s five foot nine and has mediocre ball skills, sure, everybody’s some kind of scoring guard. But play a pickup game against a six-nine guy, and suddenly somebody on your team understands what it means to be the center.
Positions are semi-important for zone defense–one defense that distinguishes forwards/centers from guards is in a 2-3 zone–which generally tries to prevent penetration to the basket. The two guards generally guard the people in the area between the three-point and free-throw lines (and should be quick enough to switch to whoever has the ball), the forwards guard the two sides of the court closer to the baseline (they do not tend to be as fast, since the area they must cover is smaller), and the center stays in the paint.
OK so what is the difference between a small forward and a shooting guard? What is the difference between a center and a power forward?
Well, positions are defined by the team, so it isn’t quite like there’s a universal standard, but in general, a center is bigger (weight more than height, really) and plays with his back to the basket in the low post, while a power forward is expected to be able to play in the high post and play on the move a bit more, and ideally actually hit a jumper every now and then, at least. So you’ve got Shaq/Dwight Howard/Eddy Curry vs. Chris Webber/Karl Malone/Elton Brand. This isn’t to say that teams couldn’t/wouldn’t/haven’t used a guy like Webber as the center, but in terms of prototype, the center is the guy getting the isolated post touches and the power forward is the guy doing the pick and roll/pick and pop and playing the high post.
As far as the 2 (i.e. shooting guard) vs. the 3 (small forward), it’s very similar and a lot less rigid. It’s mostly about size, I guess. A 3 is bigger, a better (or more versatile, at least) defender, and better around the basket, while a 2 is where you’d generally put a great perimeter scorer who can handle the ball and make a 3 but doesn’t have quite the same bulk to play around the rim. Which isn’t to say that a 3 who can shoot is bad, or a 2 who’s a great defender is bad. A lot of times you just have a bunch of guys who can play on the wing, and you use them more or less interchangeably, it’s true. But, you know, you wouldn’t call Ray Allen a 3, and you wouldn’t call Larry Bird a 2 – the skillset of each would seem “out of position,” because you don’t have many 6-4 or 5 small forwards who don’t rebound or defend, and you don’t have many 6-9 two guards who average 11 rebounds a game.
I do know what you’re talking about, because in most cases a guy isn’t clearly this position or that. A lot of players can just do a lot of things, so the position, if you look at it as a limitation, seems stupid - Chris Bosh, Tim Duncan, Garnett, Nowitzki, or, like, Boris Diaw. But that isn’t to say that very much of the time there isn’t a meaningful definition for the terms.
Also note that I’ve been talking almost exclusively about the NBA, which makes a slight difference. The difference between a two guard and a small forward is pretty much nonexistent at say the JV high school level and becomes a lot more significant at the very high levels.
There are a lot of hybrid guys in the league now. Nowadays, any distinction aside from frontcourt and backcourt players or post and perimeter players is largely meaningless, and I don’t try to think too hard about what defined roles each position should have. Take a player like LeBron James. He can run the point, he can roam the perimeter and shoot jumpers, he can slash inside towards the basket, and if he ever adds a post up game he’ll be even more ridiculous. When the Cavs run small they play him at power forward. Defensively he can guard guys out on the perimeter or he can roam around the paint to block shots and grab rebounds. He can play every position but center. What position is the best fit for him? Depends on what kind of offense the Cavs are looking to run. Players can even have one position on offense and essentially play a different position on defense. LeBron may be playing the three on offense, but if they’re playing the Lakers he’s guarding Kobe Bryant on the perimeter.
Dirk Nowitski is another good example. Big, slow, lumbering guy who’s a lights-out shooter and not a great defender. 20 years ago a player like him probably never would have existed.
My only addition is that positions can matter on defense. Not that you absolutely need to follow the traditional line-up, but assuming the other team has a classic line-up, you’ll want a slightly comparable group defending them. If you’re playing four big guys and one guard, then one of their guards is going to be defended by a big guy, who will be too slow to stop penetration and too slow to get out and stop open threes. And if you’re playing four Iversons and one big guy, they’re going to have their power forward posting up all night.
So in order to defend effectively you’ll want to have a mix of big and quick, and the one-five positions are a way of organizing that.
(Defending a different position is fairly common for Power Forwards/Centers, where often the PF defends the opposing Center)
Hmm interesting. Sounds like it is as nebulous as I thought! 
Most sports have very rigidly defined positions and people don’t switch around very much, or if they do people make a big deal about it like when A-Rod switched from shortstop to third base because of jeter. Anyways, thanks for the clarification!