At first, the player clicks on the cookie on the left side of the screen to earn cookies per click, initially at a rate of one cookie per click. With these cookies, the player can purchase items that automatically make cookies for them, such as grandmas, farms, and cursors; and the player may upgrade the number of cookies produced per click. The more cookies produced, the better items that are available for purchase. The player is progressively able to purchase better items and make cookies at a faster rate. New items and features are added with ongoing updates of the game.
Source: Wikipedia, "Cookie Clicker", available under the CC-BY-SA License.