The .wav extension actually covers quite a few different file formats. The most commonly used one is pure, uncompressed PCM (Pulse Code Modulation) formatted audio, using a range of sample depths and sample rates, but a .wav file could also be compressed using A/mu-law, various varieties of ADPCM, or any other format supported by the Microsoft Audio Compression Manager.
Wav files comply to the RIFF standard, and can include extra information other than audio data. Meta-data in the form of text, or even a small bitmap, can be embedded in the file in a form similar to the ID3 tags used in the MP3 file format. A wav file can also include starting and ending loop points, and cue points to allow the user to skip quickly between different parts of the file, or to repeat a section of audio.
A wav file is made up of chunks, each of which is composed of a header and a data section. Each chunk can contain a different type of data, or use a different compression format. Programs which don't understand the format of a particular chunk should, in theory, totally ignore the chunk, and only read or change data which is in a format they can cope with.
Wav is primarily a Windows file format, having been jointly developed by Microsoft and IBM. From very early on, Windows has included a utility to play, and record wav files, in the form of Sound Recorder, and wav files are used by Windows for event sounds, audible warnings and so on.
Most audio programs (at least on the PC) will play back wav files, and any audio recording and manipulation program worth its salt will be able to save in this venerable, but still useful, file format.