Do you have to use a codex in warhammer 40k?

Answer

No.

But it adds more ablilities to the game for the race you have it for, also they include some cool stuff about the history of the race and things like that.

Answer

Yes or at least have access to one.

Not having a codex is like showing up to a sports game with only knowing half the rules and with too few players. The Codex tells you how many points things are and what the limits are on what you can take. It tells you how you can build your army, and since your army is half the game that is kinda important.

You can get a reference sheet from the games workshop website, go to http://www.games-workshop.com/gws/content/article.jsp?catId=cat410004&categoryId=1000019§ion=&aId=3400013 and choose your team from the list. if the link dosent work go to the website and click on 'gaming' then go to articles, the downloads and finnaly 40k reference sheets. each one is free and shows every armys attack, armour save. etc, it saves alot of money