The alpha characters on a keyboard are all of the standard letters that are used in language, A through Z. Alphanumeric is a term that refers to both the letters and numbers on a keyboard.
Computer keyboards are available in a variety of character sets depending on the region for which the keyboard is to be sold. Most people are familiar with the standard U.S. keyboard style with the 26 characters of the English alphabet in what is known as the QWERTY arrangement, and numerals 1 through 0. Some versions of this keyboard style feature a number pad to the right of the alpha keys arranged in a way similar to desktop calculators. Keyboards in Asia and regions that don't use a Latin-based alphabet are designed with local characters and alphabets in mind.
Other keys on a keyboard perform different functions depending on the operating system used. The function keys labeled F1, F2, F3, etc. can be used by a software developer as a shortcut key within a program or by the operating system itself as a shortcut. A common function key combination in the Windows operating system is the Alt key and F4 key pressed simultaneously to close a window.