An overview of the capture filter syntax can be found in the User's Guide. A complete reference can be found in the expression section of the tcpdump manual page. Wireshark uses the same syntax for capture filters as tcpdump, WinDump, Analyzer, and any other program that uses the libpcap/WinPcap library. If you need a capture filter for a specific protocol, have a look for it at the ProtocolReference.