Message levels (or in short, levels) are used almost everywhere. They describe what kind of messages we're dealing with. Here's a list of them all:

    CRAP            - Can be almost anything
    MSGS            - Private messages
    PUBLIC          - Public messages in channel
    NOTICES         - Notices
    SNOTES          - Server notices
    CTCPS           - CTCP messages
    ACTIONS         - Actions (/me) - usually ORed with PUBLIC or MSGS
    JOINS           - Someone joins a channel
    PARTS           - Someone parts a channel
    QUITS           - Someone quits IRC
    KICKS           - Someone gets kicked from channel
    MODES           - Channel mode is changed
    TOPICS          - Channel topic is changed
    WALLOPS         - Wallop is received
    INVITES         - Invite is received
    NICKS           - Someone changes nick
    DCC             - DCC related messages
    DCCMSGS         - DCC chat messages
    CLIENTNOTICE    - Irssi's notices
    CLIENTERROR     - Irssi's error messages
    CLIENTCRAP      - Some other messages from Irssi

And a few special ones that could be included with the levels above:

    HILIGHT         - Text is highlighted
    NOHILIGHT       - Don't check highlighting for this message
    NO_ACT          - Don't trigger channel activity when printing
                      this message
    NEVER           - Never ignore or log this message