In Bridge, What's the Difference Between the Blackwood And Gerber Conventions?
Blackwood is used for suit contracts, while Gerber is used for Notrump contracts.  This was a necessary evil based on how Blackwood develops.  

Say we have established that we should be in Notrump, and I want to try for slam.  If I bid 4NT Blackwood, and find out that we are missing 2 Aces, how can I sign off?  5NT would ask for Kings, and 6NT is obviously too high.  There's no way to bid a safe level of Notrump anymore.  By using the Gerber 4C bid, I can sign off in 4NT if we are missing two aces.