Is the ace high or low?
High — with exactly one exception. The ace is the top rank in pairs, kickers and flushes, but in the A-2-3-4-5 straight (the "wheel") it plays low, making that the weakest straight in the game.
Where the ace is high
Almost everywhere. A pair of aces beats every other pair. An ace kicker wins the tie. The ace-high flush is the nut flush. And 10-J-Q-K-A — broadway — is the highest straight in poker.
The one exception: the wheel
In A-2-3-4-5 the ace fills the spot below the two, so the wheel is a five-high straight — the lowest one. Any other straight beats it: even 6-high. It's still a straight, though, and it beats three of a kind like any other.
No wraparound
The ace sits at either end of the ladder, never in the middle: Q-K-A-2-3 is not a straight — just ace-high. If someone at your home game claims "round the corner" straights, they're inventing a different game.
Check yourself
Which hand wins?