from the apt-get manual page:
A specific version of a package can be selected for installation by
following the package name with an equals and the version of the pack-
age to select. This will cause that version to be located and selected
for install. Alternatively a specific distribution can be selected by
following the package name with a slash and the version of the distri-
bution or the Archive name (stable, testing, unstable).
you can find out all you might want to know by entering:
hope this helps.