Gain is a term you’re likely to meet in video rather than stills photography. It basically means turning up the input signal strength to record a decent value. Videographers are more likely to talk about increasing the ISO setting rather than the ‘gain’, though it amounts to the same thing. It’s still used for audio recording, where your camera or sound recorder will probably have a ‘gain control’ or some kind of ‘AGC’ – automatic gain control.
This is a small microphone designed to attach to a speaker’s clothing for interviews or presentations, for example. They’re usually small and unobtrusive, they’re hands-free and they help exclude other background noises. They may also be called lapel mics. Some are connected to the camera or sound recorder by wire, others work wirelessly.
A shotgun mic is a highly directional microphone usually used to capture audio from a subject a little way from the camera. They’re popular for on-camera use when it’s not possible to place a microphone on or near your subject. Shotgun mics are ‘unidirectional’, which means they capture sound from one direction only over quite a narrow angle. This makes them more effective at excluding unwanted background noise.
A feature on some microphones that attempts to cut out loud roaring, whistling noise that you might not notice when shooting but which spoils the sound quality. It can be effective, but it’s even better to use a muffler on an external microphone.