If you are on reserve you are not guaranteed to fly any amount of hours per month. Typically reservists fly less than line holders unless you are given a full trip each reserve block.

As a line holder you get an assigned set of trips for the month which usually equals a higher amount of credit/pay than on reserve. (Most lines are worth more than your 75 hour guarantee)

As a reserve pilot, you still know which days you will be working, but you have know idea what is in store for you each of those days. You could be sitting at home/in the crashpad and then all of a sudden you have to be at the airport in 2 hours and wont be home for 4 days. Minus this uncertainty and the slightly lower pay, reserve at home can be quite nice. You will definitely be in your own bed a lot more and have more face time with the family.

As for choice, bidding for the month usually consists of listing the lines of flying you want in order of preference and you get what your seniority can hold. The same goes for reserve schedules which just have blocks of reserve days instead of trips.

If your company uses PBS then you input your "preferences" for how your would like your schedule to look for the month (e.g. I want this day off, I want as high a credit line as possible, I want as many days off as possible, I want day trips etc.) and the computer builds a schedule just for you based on the trips available when it gets to you.

Hope this helps, its a lot to take in but you will catch on quick. PM me with any more questions.