再 is for future repetition ('do it again later'), while 又 (yòu) is for past repetition ('did it again'): 他明天再来 'He'll come again tomorrow' vs. 他又来了 'He came again.'
mistakes
Don't confuse 再 (zài, again) with 在 (zài, at/in). They sound the same but have completely different characters and meanings.
