What is the difference between a KeySpace and a KeySpaceDefinition in Hector?

Hi:

I'm relatively new to the world of Hector and Cassandra.
I'm familiar with the concepts of KeySpaces and ColumnFamilies.

What is the difference between the KeySpace and the KeySpaceDefinition types?
Are they interchangeable?

Two methods are exposed via the Factory:
CreateKeySpace()
and
CreateKeyspaceDefinition().

What is the difference between these two calls and when would I use them?

If I want to execute a CQL query, I need to use a KeySpace.

CqlQuery cq = new CqlQuery(keySpace, .., .., ..)

How do I retrieve a existing "KeySpace" from a cluster?

Thanks,
JohnB