UNIT_SPELLCAST_SENT and _START

If possible, I'd like some clarification on the difference between UNIT_SPELLCAST_SENT and UNIT_SPELLCAST_START. Is _SENT fired when the client sends the spell request off to the server and _START is fired when the spell actually begins to cast?

I tried searching through this forum for UNIT_SPELLCAST_SENT but the forum search complains about it not being an alphanumeric search, so sorry if this has already been answered.