An omnidirectional (omni directional or omni-directional) microphone (such as the ME 2) microphone has a spherical pickup pattern. An omnidirectional microphones is good for general use when the source moves position around the microphone. With an omnidirectional microphone you do need to be aware of picking up unwanted sources and especially be careful of feedback (when live amplification is being used). An example for the use of an ME 2 would be for picking up speech for a very animated person who has a lot of head movement when speaking (ie: an actor in a play).
A cardioid microphone (like the ME 4) is a narrower pick up pattern (more up and down than spherical). A cardioid microphone is good for situations when you need to reject external sound from the rear of the microphone and especially good for preventing feedback in live amplification environments. An example for the use of an ME 4 would be for picking up speech of a person with very little head movementbut the need to reject external audio sources (ie: a news anchor at the news desk).