Electronic CounterMeasures, or ECM, is one principal part of Electronic Combat. Specifically, it is the Electronic Attack component of Electronic Warfare.
Offensive electronic countermeasures(OECM) are used to hinder enemy electronic devices. In an offensive role jamming is used to disrupt enemy radar; IE denying targeting information and creating random radar hits. OECM can be land or sea based, or emanate from dedicated jamming aircraft. The effectiveness of OECM is degraded as distance increases and the bearing spread to the target increases.
Defensive electronic countermeasures(DECM) - sometimes referred to as Self Protection(SP) are used to hinder enemy electronic devices. In a defensive role DECM is used to disrupt the radar homing on guided weapons, as well as radar performance of Target Tracking Radars. The majority of modern aircraft carry dedicated DECM pods with every load out, and most naval vessels carry powerful DECM gear as well. Distance to the victim radar makes a big difference to effectiveness - below a certain range, burnthrough occurs - the range at which the victim radar's output energy is sufficient to 'ignore' false target returns. Some radars have burnthrough modes, which can increase the range this occurs at. Relative bearing has no effect on ECM operation, although it does have an effect on Radar Cross Section and thus relative performance of ECM.