How does counting off/counting in/leading in work?

[br]I don't have a good feel for figuring out the tempo of a song unless it's stated and on top of that, there seem to be variations in how it's done.

[br]i.e. One / Two / One Two / One Two Three Four.

[br]Also, if a song has a pick up note on say count Four, is there a way that's reflected or is that pre-discussed with the band?

[br]Thanks!