Noise jamming — make the radar deaf
The simplest jammer broadcasts loud broadband noise on the enemy radar's frequency. The receiver's noise floor rises until real targets disappear. Effective but obvious: the radar knows it's being jammed, knows roughly where the jammer is, and can shoot a missile straight down the jamming beam (home-on-jam mode).
Deception jamming — make the radar lie
More elegant. The jammer listens for the radar's pulse, then transmits a copy with a small time delay so the radar sees a false target slightly behind the real one. By gradually increasing the delay, the false target is 'walked off' and the radar locks onto empty air. Modern Digital Radio Frequency Memory (DRFM) jammers can do this for dozens of radars simultaneously.
Chaff and decoys
Chaff — clouds of aluminium strips cut to half the wavelength of enemy radar — was invented in 1942 (codenamed Window). Modern chaff is the same idea with better metallurgy. Towed decoys like the ALE-50 trail behind the aircraft, broadcasting a stronger version of the aircraft's radar reflection so missiles chase the decoy.
ECCM — the radar fights back
Electronic counter-countermeasures (ECCM) include frequency hopping, low-probability-of-intercept waveforms, monopulse angle tracking that can't be deceived by amplitude tricks, and AESA radars that can null out jammers spatially. The cycle never ends: every new ECM gets a new ECCM within a few years.