What's the relationship between linux-next and mainline

[wuzhouhui]

Hi,

Frequently, I found Stephen Rothwell send email about he have created 
today's linux-next tree.
On the other hand, the maintainer sometimes write their pull request 
like "these changes have
been sitting in linux-next for a while".

So, what's the relationship between linux-next and mainline? Is 
linux-next a temporary integrating
tree for maintainers to merge and test?

Thanks.