A stream is a UCM term, in base ClearCase, you don't have any streams - only branches.
A stream is very tightly connected to a branch and conceptually you can say that a stream is a wrapper around a branch.
When you create a stream in UCM ClearCase automatically creates a branch underneath it, the branch and the stream will have the exact same name upon creation, and only if you go and manually rename either the stream or the branch will the names diverge.
The stream and it's branch are connect through a ClearCase hyperlink called
As seen in the example above, the stream is pointing to the branch type, and the branch type is pointing to the stream.
Streams comes in two flavours: Those that are integration streams - and those that are not - which technically all are development streams.