On road courses, qualifying takes place in two sessions over two days on the Friday and Saturday before race Sunday. Each session is split into two halves, the slow half and the fast half. All drivers in each half can go out any time they want to, so more than one car will be on the track at a time.
Timing is handled electronically by a sensor in the car.
On Friday, slow and fast drivers are determined by order of finish in last week's race. Drivers are ordered on the grid in order of the fastest time they were able to turn in their qualifying sessions. Thus, the pole winner will always have turned the fastest lap, unlike the Indy 500 where this is not guaranteed. There's a couple of other contingency rules for determining the IndyCar grid. If Saturday qualifying gets rained out or for some other reason isn't held, the grid is determined based on Friday's qualifying times.