Mana is what you get from mana sources, so you're asking wether or not you HAVE to have Lands in your deck, right?
The answer is no, you don't have to, but it's awfully hard playing without them, unless you're playing Vintage. So, a short rundown on Lands and deckbuilding:
- Lands DO count against the maximum cards for a given deck;
- Lands ARE subject to the Rule of 4, except Basic Lands (there are 10 types of those), which you can have in any amount in a given deck;
- You CAN have Lands in a sideboard;
- A deck should USUALLY run about 22-24 lands in Extended and Standard, and although there ARE exceptions to this, they should be few;
- A Limited deck should run 17-18 lands, although I've seen them ranging from 15-18 with reasonable success.
So, any more questions?
PS: Went through a Field Test, "judging" the Shadowmoor Pre-Release, around 70 people. So, well, it seems I'm sharp on rules questions, so fire away!