Every airline is different and every airline/pilot contract is different. In general, if you hold a Reserve Line for the month, you're "on-call" most days during that month. In general, if you hold a Line for the month, you've got a schedule for the entire month and can trade trips to change your schedule as you see fit (provide it complies with contract/FAR limitations).
Pilots "bid" by seniority for each monthly (or whatever) schedule. The more senior you are, the better your chances of getting the schedule you want. What you bid is whatever you want. At AA I still see many very senior pilots (#1 or #2 in bid status) bidding Reserve because that is what they want. Personally, I prefer any SNA schedule I can hold to the uncertainty of Reserve duty or the LAX schedules. Last month I held next to last SNA line (25 of 26). Next month I've got my third choice (3 of 27). I'm still about 42nd of 60+ in my bid status. Go figure.