A left-click involves clicking the left mouse button. Typically, "left-clicking" means the same thing as just "clicking" since the left mouse button is the primary button by default. The term "left-click" is most often used in contrast to "right-click," which involves clicking the secondary button on the right side of the mouse.
Left-clicking is used for many common computer tasks, such as selecting objects, opening hyperlinks, and closing windows. Clicking and holding the left mouse button can be used to select text or perform drag and drop operations. In video games, left-clicking is typically used to perform the primary action, such as moving a character or firing a weapon.
For right-handed users, the left-click is the most natural click, since it is done with the index finger. For people that use a mouse with their left hand, it is a bit less natural since left-clicking is done with the middle finger and the index finger is used for right-clicking. Therefore, Mac OS X and Windows allow you to switch the primary mouse button from left to right.
NOTE: Double-clicking is simply two left-clicks in rapid sequence.