I dont understand the differences betwees PSUs. I know that they vary in different wattages and stuff but could some one talk me through on why you need different wattages and stuff ?

Like what would happen if i was using, say umm, a 400 watt PSU then switched to a 600 watt PSU. What would happen ?

Likely nothing would happen - if your PC came with a 400W PSU, then it's probable that the hardware inside the PC doesn't draw more power than the PSU can deliver - 400W.

Adding more wattage to that will not substantially change the operation of the computer unless you have installed some power-hungry hardware - like a new graphics card - and that hardware isn't working properly because it can't draw enough power from the PSU. This will manifest itself in very poor framerates in games, for example, since a graphics card draws much more power when rendering 3D objects intensively.