master and origin

Hi all,
I went trough the docs I found on the web but I still don't fully
understand why if I clone a remote repository my local copy has two
branches, origin (that is always a exact copy of the remote
repository) and master which is... what? The branch supposed to be
used for local development?

I'm used to just checkout to a new branch, do my own development and
then diff against origin so I'm missing why I see the master branch.

I'm sure I'm missing something very fundamental but I cannot figure
out what is it :-)

Thanks!

regards,
-- 
Paolo