In card games the playing card with a capital "A" on its face is called the ace. Usually it has a large picture of card's suit in the middle.
Depending on which game you are playing the ace can be considered a high card or a low card. In Rummy and Gin Rummy it is played as a low card. In Spades and Bridge it is played high. In poker games you can use the card at the high or low end of a straight. Also, in "low poker" the ace is usually considered to be lowest card.