For reference, here is a summary of different qualifying systems used (as far as I can remember them!):
1995 and before: 1 hour on Friday, 1 hour on Saturday, 12 laps in each session (including in and out laps), fastest lap overall counted. Low fuel.
1996-2002: 1 hour session on Saturday, drivers limited to 12 laps. Low fuel.
2003: Single-lap shootout on Friday (canít remember how order was decided) to determine order for Saturday. Single lap shootout on Saturday for the grid. Race fuel on Saturday.
2004: First shootout was now on Saturday, order decided by finishing order from previous race (winner going first). Race fuel in final qualifying.
2005, first 6 races: 1 lap on Saturday, 1 lap on Sunday, grid position decided by total of two laptimes. Low fuel on Saturday, race fuel on Sunday (?).
2005, rest of season: Single lap on Saturday with race fuel, order determined by finishing order in previous race (winner going last).
2006, initially: 3 sessions of 15-15-20 minutes, losing 6 cars in each first 2 sessions (on low fuel). Final part took place on race fuel, with drivers receiving the fuel back that they had used in Q3 (provided the laptime was fast enough). In first 2 sessions, laps only counted if they were finished within 15 minutes.
2006, subsequently: Final part reduced to 15 minutes, laps only had to be started before the flag in Q1 and Q2.
2007-09: Q1 extended to 20 mins, Q2 reduced to 10 mins. Fuel not returned after qualifying.
2010: Q3 became low fuel, drivers have to start the race on the tyres with which they set their fastest lap.