|Status: This article is about a deprecated MusicBrainz technology.|
TRM (TRM Recognizes Music) IDs are (somewhat) unique ids that represent the audio signature of a musical piece (see audio fingerprint). They are simply IDs in Relatable's database and can't be used to find any information about a song, and effect of this is that fuzzy matching (songs that "sound similar") is not possible with TRM IDs.
In order to determine a TRM ID an application such as the MusicBrainz Tagger can send data from a music file to the TRM server, which will reply with a TRM ID. The TRM ID can then be used to look up the metadata for that song from the MusicBrainz database. For more information Relatable has a detailed article.
There is a utility called "trm" included in the Tune Pimp distribution that can be used to calculate the TRM of a music file. One use of this is to find duplicate music files.
The TRM system has several well known flaws, and this has led to some discussion about abandoning the TRM system completely. For more on this, see future proof fingerprint.