It can be tricky deciding when to prune or cut back shrubs and trees. There are some where it means no flowers next year if if you do it too late after flowering. As you don't mention any particular type of shrub or tree. Either use the alphabet at the bottom of this screen or type in name of shrub + prune into google and an answer should appear.

Depends on the tree or bush - as dorjac says, pruning at the wrong time can cause problems, so you need to know which plants you've got first.