Most shows[1] that are notMickey Moused have a stock set of instrumentalBackground Music (BGM) to use as themes for particular characters or sequences, or simply to set a mood. These usually fill up the bulk of a show's soundtrack, aside fromTheme Songs and (in the case ofAnime)Image Songs.
The pieces that are most closely tied to characters or events areLeitmotifs.
For when it's not so much in the background, seeLeft the Background Music On.
See alsoBackground Music Override,Variable Mix,Diegetic Switch.
Specific instruments andStandard Snippets are used to create aMood Motifs.
For further exploration of this topic, listen to Anna Russell's "The Ring of theNibelungs (An Analysis)".