The raise the stats by 50%, which is why they are popular competitive. However, as you'd probably know, they will only allow you to use the same move in a row, unless you switch out or the item is removed.
- Choice Band increases Attack by 50%.
- Choice Scarf increases Speed by 50%.
- Choice Specs increases Special Attack by 50%.
Source: Bulbapedia's In-battle effect item page. The same source for the following:
For the Life Orb, it increases the power of the move by 30% (special or physical), though at a cost of 10% of max HP with each hit.