Classic solitaire (also known as Klondike Solitaire) is the game that many simply refer to as "solitaire". Klondike is by far the most popular version of Solitaire, though there are many variations. If you did not realize there is more than one type of solitaire game, the game you know as Solitaire is likely to be Klondike.
Classic Solitaire is a game of sorting cards. You move cards between columns in an attempt to put them in order into 4 piles of cards separated by suit. The game begins with 28 cards dealt into columns. This is known as the tableau. Seven cards are dealt in a row-one card face up, then six more continuing to the right face down.
Next, deal a card face up on the second pile, then one more in each pile facing down. Continue in this fashion, dealing one less card each time, until you have seven piles that start on the left with one card and increase by one card with each column from left to right. The top card on each pile is facing up. Each time an Ace appears face up, place it in a row at the top. These are the foundations. The remaining 24 cards are placed in the top left of the game screen as a stock pile you can draw from when you need additional cards.