Most of the question was answered above. At alot of companies there are two types of reserve.

Home reserve. You have somewhere between 1 and 2 hours from the time you get the call to you get to the airport/plane (depending on what contract says). So if you live in base this is cool because you get paid to be at home.

Airport/Hot/ARC/Ready...You have to sit at the airport and wait on a phone call. There usually isn't much notice (15 mins) between call to the flight.

Pay. Most companies have min pay of 70-75 hours so you know you can sit home reserve all month and get paid min pay. If you fly most companies have a min pay credit per day. You get paid that min credit or block for that day which ever is greater (if thats in your CBA) Same thing is said for airport reserve. You get a min day credit and if you fly you get min credit or block for the day. To figure out your pay you just add up your daily credits. If daily credits are above min pay of 70-75 you get paid what you flew/credited. If it adds up less than min pay of 70-75 you get paid min pay. I'm not sure how it is at all companies, but at mine reserve guys fly or sit airport reserve all of the time so pay credits are normally 90-100 hours.