Aces may indeed be used as low in a straight in holdem. A2345 is the lowest possible straight, also sometimes called a "wheel." In any other context, aces will be ranked higher than other cards.
For example, a pair of aces beats a pair of kings (considered to be the higher pair). If you have the hand A-2 3 4 5 then it is low (a one value) to make the straight. In High poker, aces are whatever they are most advantageous to be. That is, they are always high unless they help you form the straight a-2 3 4 5.
