Definition of straight
A straight is when a player forms 5 consecutive cards. The smallest possible straight is A-2-3-4-5 and the highest is T-J-Q-K-A (10, jack, queen, king, ace). The cards do not have to be the same "suit", i.e., the straight does not have to be all spades or diamonds, etc. The cards do not have to be in the same "suit". If a player makes a straight of the same suit (say, 5-6-7-8-9 all check) then it is said to be a straight-flush and not a straight. The straight is stronger than a three of a kind and to beat a straight, you will have to get a flush or better.
Other considerations on the straight
On the Hold’em, when the main board (the community cards) shows a straight, all players will have that straight. If a tableau is 5-6-7-8-9, all players, regardless of their hidden cards, will be credited with that straight. If a player has a 10 in hand or 10-J, he will now have a higher straight and will be the one to win the jackpot. In Omaha, the operation is not the same. If we are talking about the same table as before, 5-6-7-8-9, no player would have a straight since in Omaha, only 3 cards from the table and his two private cards must be taken.