What does cannabis do?

Depends on what you mean
The immediate effects: you will get high; you can hallucinate, get giggles, munchies etc. You may become philosophical or become focused on something, then again you may just drift off and day dream. Things may feel great by touch, you may get hot or cold or tired (this can be positive cos you have a great sleep but if you want to stay awake then its a negative). There are some negatives such as dry mouth. Mianly, you become a happier, more sociable person especially compared to someone who is drunk and making a fool of themselves or being violent.
Long term affects: gateway drug theory - some believe it is a gateway drug however most go on to harder drugs because of the crowds they hang around with. Addiction theory - cannabis is less addictive than chocolate or caffeine however many claim you can be addicted to it, this is because of the tobacco people smoke in spliffs with it. Apparently it can alos affect your motivation but that affects people who smoke it all the time; personally, i find the occaisional spliff before a task makes it far easier.