Definition of straight
A straight is when a player makes five consecutive cards. The lowest 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 all diamonds, etc. If a player makes a straight of the same suit (say 5-6-7-8-9 all diamonds) then he is said to have a flush, not a straight. A straight is stronger than a three of a kind, and to beat a straight you will need to make a flush or better.
Other considerations on the fifth
In Hold'em, when the main board (community cards) results in a straight, all players will have that straight. If a board is 5-6-7-8-9, all players, regardless of their hole cards, will be credited with that straight. If a player has a 10 in his hand or 10-J, he will now have a higher straight and will be the one to win the prize pool. In Omaha, it doesn't work the same way. If we're talking about the same board as before, 5-6-7-8-9, no player would have a straight because in Omaha you only need to take 3 cards from the board and your two hole cards.