As you can see in the minimap above there are three lanes. These are commonly reffered to as 'Top' 'Mid' and 'Bot'(bottom), which is something you're probably familiar with. The image above also illustrates how the running distance to and from the lanes are different for each team. The mid, top and bot monikers are pretty obvious.
Less known to noobies, is that the top and bottom lanes are also referred to as the 'Easy' lane, 'Hard' or 'Suicide' lane. The reason for this is because of the length of the lane and the opportunities for ganking from the jungle is very different on either side. As Atlas kindly pointed out, this difference is different for both The Dire and The Radiant. Basically, the Easy lane, is the lane where your jungle is next to your tower, and the Suicide lane is the lane where your tower is next to the secret shop.
Radiant: Bot = Easy, Top = Hard
Dire: Bot = Hard, Top = Easy
Now I'm not an expert(yet) but these are the basic guidelines I've picked up so far.
Because of the danger in the Suicide/Hard lane, you want to have heroes here that have strong escape abilities, or skills that lend to their ability to avoid ganks. Typically you'd want to put some kind of support here to ward the jungle and support the lane, and some kind of semi-carry with escape mechanisms.
Windrunner is a strong example of a great hard-lane hero. She has a ranged stun, and Windrun, which basically lets her get out of any early lane situations. Characters with stealth like Riki or Bounty Hunter also make for strong options.
The hero that goes mid should be any character that has level dependent ganking potential. Being melee or ranged isn't as much of a factor as 'can this character leave lane and contribute to a successful gank at top or bottom.' This is why Dragon Knight, Pudge, and Night Stalker, should go mid, even thought they're not ranged. At level six (or in night stalker's case, night time) you have great ganking potential.
Sometimes you will see a support/semi-carry go mid, often a ranged character with a good disable. This is a perfectly good strategy and something we saw a lot with Rubick play in The International 2. Over all the main thing to remember is to have a character mid that has ganking potential based on level dependency.
Easy Lane is where you always always always want to put your hard carry. Whatever character you have that isn't going to be of any use unless they get farm and levels. Phantom Assassin Faceless Void and Drow Ranger are prime examples of carries that would benefit from the relative safety of the Easy lane. If you have only one support on your team, send them to the Easy Lane with the carry. The support should focus on denies and harrass while letting the hard carry get as many last-hits as possible.
One of the only exceptions to this rule is in the case of Anti-Mage. Because of his Blink ability he can safely operate in the suicide lane while also farming and chasing heroes with his Blink when needed. In general though, remember that the hard carry usually goes to the Easy lane and you can't go wrong.
I hope this has clarified a few of the basics of lane composition. This is just the broad strokes and it can obviously get more detailed based on individual heroes chose and overall team strategy, not to mention tri-lane and jungle team comps. However, for low-level pubs, this should help avoid some confusion on how the lanes should be set up, and which lane you should head to in order to be most effective on your team.