i want to know exactly when i should use either of commit, update and merge command in svn.

after i've checked out a project and altered the code, should i use update, commit or merge to stay in sync?

correct me if im wrong:

update = all changes in the repo is copied to your local project.

commit = all changes in your local project is copied to the repo.

merge = same as above, but you determine the direction?

when do i use each command above?