FOURCC is short for "four character code" - an identifier for a video codec, compression format, color or pixel format used in media files.
A character in this context is a 1 byte/8 bit value, thus a FOURCC always takes up exatly 32 bits/4 bytes in a file.
Another way to write FOURCC is 4CC (4 as in "four" character code).
The four characters in a FOURCC is generally limited to be within the human readable characters in the ASCII table. Because of this it is easy to convey and communicate what the FOURCCs are within a media file.
AVI files is the most widespread, or the first widely used media file format, to use FOURCC identifiers for the codecs used to compress the various video/audio streams within the files
Some of the more well known FOURCCs include "DIVX", "XVID", "H264", "DX50". But these are just a few of the hundreds in use.
To find out which FOURCCs are used within a media file, you need to use an application specialized to open and inspect the media file.
In our fourcc identifier section we have several such applications (all free) available for download.
You may refer to our video codecs for a long list of which FOURCCs that identifies which video codecs.
For audio codecs it is not FOURCCs that is used, but rather audio tags, or an audio identifier - that identifies one specific audio codec or one type of audio compression scheme.
An audio tag is just an integer decimal value (32 bits) or alternatively often specified as a HEX value. Your best bet at locating an audio codec given the audio tag is probably through this list.