In a custom map that I saw on youtube ( http://www.youtube.com/watch?v=kjSZcxv22fA ) I saw glass that they would jump on and break through, how can I do this also?

Func_breakable.
Find a glass texture you like, and make the brush. Then rightclick and choose 'Tie to entity'. In the dropdown list in the window that pop up, find "Func_breakable". The default material should be glass. Then go to flags and check "Break on touch". And that is about it.

(P.S. it's a long time ago i've used hammer, so i may have skipped something, or said something wrong. I've been using the GECK for Fallout 3 instead )

Alternatively, you can also use a func_breakable_surf, which shatters more realistically and is designed specifically for glass. There's only one texture that's designed to work with it, though, glass/glasswindow_frosted_break001a.

There's no "break on touch" flag, but you can easily set up a trigger_once that will break the glass for you.

Also, you can set up a "pressure delay" for increased realism, which will crack the glass before completely breaking it (in the keyvalues of the func_breakable_surf).

Happy Mapping!