More information can be found on the Help Page. Short summary below.

Basic boolean operators:

• AND: fender & rednef -or- fender rednef
• OR : fender | rednef
• NOT: rednef -fender -or- rednef !fender
• Grouping: fox & (box | socks)

Capabilities

• Field searching: @title fender @message rednef
• Phrase searching: "quick brown fox"
• Word proximity searching: "quick brown fox"~3
• Quorum matching: "red_fox gray_fox arctic_fox fennec swift_fox kit_fox"/2

Available Fields

• @title
• @message
• @lower (artist name as it appears in their userpage URL)
• @keywords

Example:
(fender | rednef) -truck @keywords plushie