There is no difference between Bokura and Bokutachi. And Both are male term. Boku is the word for young boys,often used in schools. Usually an adult male does not say Boku. Young girl could use Boku, but it means she intentionally uses a male term. Adult male could use Boku, but it means he is still so young that he intentionally uses a word for young boys.

I learnt these two words from Japanese catoon and games. Go to see and play them~

I think Bokura is more a male term, and Bokutachi is more general.