What is the minimum re-raise in Texas hold em? I have heard both that you need to double the previous bet and that the minimum raise is the big blind. Which is it?
In limit games, the only raise amount allowed is the big blind during preflop and flop play; double the big blind during turn and river play.
The correct minimum raise in big bet games (no limit and pot limit) is to increase the amount of the bet by the amount of the previous bet (e.g. double the previous raise*), or to raise all-in if you do not have enough in your stack to actually double the previous raise. Reference: http://www.learn-texas-holdem.com/how-large-is-the-minimum-raise.htm
Note that doubling the previous raise is different than doubling the previous bet. If I were to bet 100 and you raised to 300, someone could raise to 500, even though it would take 600 to double your bet.
In the case of an all-in, there is a distinction between what it allowed for the raising player, and what constitutes a raise to the rest of the players at the table. If a bet is considered to be a full raise, it reopens the betting, allowing another player to raise again if they choose. In the case of an all-in for less than the minimum raise, the all-in is allowed, but it does not constitute a full raise, and as such it does not reopen betting. In many cases, betting will reopen for an all-in raise that is a fraction of a full raise as well, though this varies a bit from casino to casino. Typically, if this is the case, the most common amount required to reopen betting is 1/2 of a full raise.
As noted in the comments on the question, one place where the definition of what constitutes a minimum raise is in the situation where a three-way pot occurs with the smallest stack acting third and shoving all-in for less than a true raise. That is: Player A raises to 300, then B raises to 500, then C shoves for 599. If A calls, then in most casinos the only options B has left are to call or fold; since no one made a full raise after his last raise, he is not permitted to raise again. Had C shoved for 600 or more, many casinos (but not all) casinos would allow B to raise again.
The OP asked "I have heard both that you need to double the previous bet and that the minimum raise is the big blind. Which is it"
The big blind is a bet, just a blind bet meaning the player bet blind before they received cards, so like all other bets, the minimum raise is always the size of the bet. You may have been told or heard that the minimum raise must be the size of the blind, what they were meaning was that the current bet size is the big blind. The comment you must "raise the size of the blind" just means that if the blind is 5, you must raise 5. It has nothing to do with the context of the blind, just the context of the size of the bet which just happens to be the size of the big blind.
There is no structural distinction or different rules concerning raises made in limit and no limit. The rule is the same for both, a raise must be the size of a bet, it does not matter if the bet size is constrained because the game has a betting limit.
There are however a wide variety of rules and confused players, dealers, and floor people interpreting those rules when the raise is all in. The general rule is that in limit, if the all in raise is half or more of the bet, it reopens the action as though the raise is a full raise. In big bet games, (Pot limit, No Limit) the general rule is that the raise needs to be a full raise in order to reopen the action.
There is a whole other set of rules for when a player goes all in with a bet that is smaller then minimum bet allowed. They are similar but different enough to add confusion to the whole process.
The rules vary from house to house. The rules are the same for most poker variants and not just Texas Hold'em. The short answer to your question is that the big blind is just a bet like any other, so the question "which is it" is rather mute, there is no which is it, they are both bets.