Source: http://www.quora.com/How-does-a-fighter-...

The primary technology that a military aircraft uses to lock and track an enemy aircraft is its onboard radar.  Aircraft radars typically have two modes: search and track.  In search mode, the radar sweeps a radio beam across the sky in a zig-zag pattern.  When the radio beam is reflected by a target aircraft, an indication is shown on the radar display.  In search mode, no single aircraft is being tracked, but the pilot can usually tell generally what a particular radar return is doing because with each successive sweep, the radar return moves slightly.