The Shift key is a modifier key on the keyboard. Its primary purpose is to capitalize lowercase letters. When the Shift key is not being pressed, letters are entered as lowercase by default (unless Caps Lock has been activated). When a user holds down the Shift key while typing a letter, the capitalized version of the letter is entered.
The Shift key also affects other keys on the keyboard. For example, you might notice that the numbers near the top of the keyboard have symbols above them. These symbols are entered when you hold the Shift key while typing the corresponding number. For example, Shift+1 enters an exclamation mark (!), while Shift+8 enters an asterisk (*). The same is true for the punctuation keys, such as period, comma, semicolon, etc. The lower symbols are entered when the Shift key is not being pressed, while the top symbols are entered when the Shift key is held down.
Since Shift is a modifier key, it can also be used in conjunction with the mouse. For example, many programs will allow you to select more than one item by holding down the Shift key and clicking multiple items on the screen. It can also be used to select a section of text by clicking in one spot, holding Shift, then clicking in another spot. Finally, Shift is sometimes used for keyboard shortcuts that perform common commands. For example, Control+S is the standard "Save" shortcut, while "Control+Shift+S" is typically used for "Save As..."