Yoda Is Your Father said:

Thanks Silicon. So wait... there are only ever 2 sith? Just two?

When Yoda said 'always 2 there are' in TPM I thought he was just saying 'If there's one, there's another out there somewhere'. I didn't realise there were ONLY ever 2 in the whole universe at any one time.

The prequels really didn't explain what a sith was, why there were only 2 of them, or what they wanted revenge for...

Here you go...if you're bored, read this

http://starwars.wikia.com/wiki/Sith_lore