There's only one "B" in German.
The other one is an "ß", called "Eszett" or, in Germany, mostly "scharf-s".
There's a simple rule when to use it: if the vowel before is short, then you use ss and if it's short, then you put ß. Examles:
- der Fuß = the foot (the "u" is long)
- der Hass = the hate (the "a" is short)
The ß is necessary to avoid confusion, too:
- die Maße = the measures
- die Masse = the mass
There has been a spelling reform in 1996 and many Germans are confused about the correct usage of the ß. However, it's not simply replaceable by two s you'll understand from what I explained above. ;-)
Native speaker of German