Basically they're all for sync'ing all the bits of recording gear in your studio. The Wordclock and Superclock ones are for digital recorders so that their A/D and D/A converters all run in sync. The SMPTE allows the records to stop and stop playing/recording at the same time as well as any shuttling functions. Say you have 3 ADAT machines with your tune recorded across all three of them - you have to sync them up so that the tune will always playback correctly. You can't press Play on all three of them at the same time can you! SMPTE can.
Footnote: Superclock isn't used that much beacuse it runs incredibly fast and is therefore easier to distort.