Qualifying decides starting position, and in a sport where overtaking is difficult, starting position often decides the race. It is run as a knockout in three segments, usually on the Saturday.
Q1
Every car takes part. The segment lasts eighteen minutes and drivers can run as many laps as they can fit in, subject to tyres and fuel.
Only each driver's single fastest lap counts. When the segment ends, the five slowest drivers are eliminated and fill the back rows of the grid, ordered by their times.
Q2
Fifteen minutes, with the remaining fifteen cars. Same principle: fastest lap counts, five more drivers are eliminated at the end and take the middle of the grid.
Q2 is often where the real strategy sits. Teams want to progress while spending as few sets of fresh tyres as possible, because tyre allocation for the weekend is limited and the sets used in qualifying are gone for the race.
Q3
Twelve minutes, ten cars, no eliminations, just the fight for pole position. Almost every driver runs two flying laps: one early to bank a time, one late when the track has the most rubber and is at its fastest.
The fastest lap in Q3 takes pole. The rest of the top ten line up behind in order.
Track evolution and traffic
Two things make qualifying harder than driving fast. The track gets quicker through each session as rubber builds up, which is why the final minute of each segment is the busiest and most chaotic point of the whole weekend.
And a fast lap needs clear air. A car half a second ahead disturbs the aerodynamics of the car behind, so teams spend enormous effort on release timing and drivers spend the out-lap manoeuvring for space. A perfectly good lap can be lost to a slow car in the final corner.
Penalties change the grid afterwards
The qualifying result is not always the grid. Drivers who exceed their season allocation of power unit components, change a gearbox outside the rules, or carry penalties from the previous race are dropped a set number of places. That is applied after qualifying, and everyone else moves up.
The sprint weekend variation
At sprint events the format changes. A separate shorter qualifying session sets the grid for the sprint race, and the main qualifying session sets the grid for the Grand Prix. The two are independent, so a bad sprint qualifying does not affect Sunday.
The short version
Eighteen minutes and five out, fifteen minutes and five more out, then twelve minutes for pole among the last ten. Fastest single lap counts, penalties are applied afterwards.