Compressors are a signal processing devices that is used to reduce the dynamic range of an audio signal. Think of it in some sense as an "automatic volume control" that is used to turn down an audio signal when it reaches a certain threshold.
Limiters are a "brick wall" signal processor that entirely stop an audio signal from going over a certain theshold. These are most commonly used to protect speakers from damage due to over or misuse.
Gates are a signal processing device that turn off an audio signal when the levels go below a certain threshold. These are most commonly used to reduce noise in a system, or prevent microphone spill from adjacent instruments.