@Winternet: I think that @Terramagi and @Sweep did a pretty good job of explaining denies. Here's a bit more in-depth information (lots of this is redundant, but there is new stuff!)
When your own creeps are in last-hit range, hit 'a' and left click on them. This prevents the enemy from getting any gold and seriously reduces the amount of XP they get if they're in XP range (like half I think).
If you aggressively attack your own creeps (ie, as soon as they get below 50%), it is definitely possible to get your lane to move backwards. This can help you move into a safer area closer to your tower. Try not to go to far, because if the creeps get in range of the tower, the tower will make short work of the wave and push the lane out.
You can also deny friendly heroes IF they are ≤ 10% health and they have a dot on them.
Towers can be denied under 10% health. It's not really ever a good idea to kill the 'tier 3' defence towers, as they protect your rax and regen health slowly. Also, killing the first tier 3 tower removes invulnerability from your base's structures.
@Cronus42: Different level of play. In very high queues, people generally have enough game awareness, so typing is more than sufficient. Also, many cammed games I watch are usually the person playing carry heroes. Which is fine, but the carry is usually the least involved as far as coordinating ganks and what not... he is either farming or making a mess of things. Even then, sometimes pros can lose from lack of communication.