How does the tenure process work

I am curious to know how the tenure granting process works in the US. How much weight is given to research, teaching, and service. I have heard that a lot depends on your relationship with your peers - department colleagues. If they feel you are a good fit, you can be granted tenure even though you have less than a stellar record.

Is there a minimum requirement for number of publications. Do publications before joining the institution count? I would appreciate your inputs.