Hey Did....
'Stand-by' is at the Compass (crew report) centre and 'Available' is a sort of home stand-by.
Stand-by is for a 6 hour block and they start every 30 mins from 05:00 to 18:00. You can be called for anything from a 'there and back' to a 3 day trip. You cannot be rostered more than one stand-by in a row.
When you have an 'Available' you call 'Voice Response' (one of those freaky computer voices) at 18:00 the day before to find out your duty (a telephone allowence is paid annually). You will either be allocated flying, stood down or rarely a 'ring-in'. If they don't have something for you at 18:00 the day prior to your available day you are not obliged to check again after that.
Hope that makes sense!