For any serious recording trip of more than a couple of hours close to home, quite a range of equipment is needed beyond the obvious basics of microphone and recorder, and all of it needs to be carefully chosen.
There's a huge variety of microphones at prices from a few tens to several thousands of pounds, euros or dollars. Almost all are designed for use under controlled conditions in the studio. But field conditions are very different.
All the microphones we are likely to encounter operate on the same basic principle. They convert the varying air pressure of sound waves into mechanical movement - usually of a thin piece of lightweight material such as plastic film or aluminium foil - and convert that movement into an electrical signal.
Microphones are primarily categorised by their pickup pattern or directional sensitivity - the degree to which they ignore off-axis sounds. They range from almost completely non-directional to narrowly directional with only about a 30° angle of significant pickup, and differing degrees of directionality are needed for different recording scenarios.
The simplest microphone is the omnidirectional (omni). An omni microphone theoretically picks up sounds from all directions with equal sensitivity.