What is the . in formula ~. syntax?

I know if I have a dataframe with columns y, x1, x2 and I wish to have y as my y value and x1 and x2 as x values I can do:
y ~ x1 + x2

or 

y ~.

but can someone explain what . actually is or what its transposed into?

I searched for this with no success, reading the "formula" manual pages.

Brian