What is the difference between capped and uncapped ADSL?

Capped vs. Uncapped Speed:

The performance of Capped accounts and Uncapped accounts differ in subtle ways no matter the company you choose to use.
Generally Uncapped ADSL runs off a higher 'contention ratio' than Capped ADSL.
This means that generally more users are allowed for a certain chunk of bandwidth in Uncapped than in Capped.
This means that the number of users downloading at any one time affects Uncapped more than Capped. At any given moment if a ton of users are busy simultaneously then Uncapped will be slightly slower than Capped.

A very simplistic metaphor for this would be:
Uncapped is like eating at a Pizza place on 'All You Can Eat' night. The speed you get your separate single slices of pizza delivered to you is dependent on how busy the restaurant is at that moment. This means it may take slightly longer to eat a whole pizza but you can eat more over time.

Capped is like eating at that same Pizza place but ordering the pizza size you want off the menu - you'll get your full pizza quicker and can eat it faster but you have a set amount to enjoy.

In essence Capped is a slightly premium service if speed is a massive issue to you while Uncapped is the way to go if solid speed is good enough and you don't want to worry about getting capped at any time of the month.

You will find that the speed of Capped vs. Uncapped most differs during office hours - Thus if speed is of primary importance to you during office hours then Capped may be the best solution for you.