source control workflow

When you use source control, do you normally check in first directly to a branch most people are working on, a publicly visible branch only normally you check in to, or a local/private branch?

Is there a protocol for checking in/merging? Code review? Tests pass? Just "don't break the build"? Nothing?

How often do you check in *somewhere*? How often do you check in/merge to a branch other people are actively using?