Pruning a tree depends on the tree and the goal you have in mind ;-) It really is a question that is a bit too broad for a simple answer.

A grafted tree is a tree that is not growing on its own roots. It can also be a tree that has branches of another tree on it. For example an apple tree that produces more than one variety of apple on the same tree. Fruit trees commonly have both done.

For a lemon tree the Improved Meyer Lemon is very popular and does well as a houseplant if you have a sunny window or supplemental lighting. Don't know anything about avocados.

A good place to find trees and shrubs suitable for containers is Raintree

You'll find a great deal of relevant info over on the Trees forum. They even have some FAQ's there that will help. And the Plant Propagation forum has info on grafting.

Dave

Here is a link that might be useful: Tree Forum

Thanks for both of these links...I hve been all over them!