Manual alphabet is used to fingerspell a series of the alphabetical letters to form a phonetic or a spoken/written word of another language. E.g. a person's name, a place, a technical word, etc.
You can download the poster (pdf) of American Manual Alphabet.
There are different manual alphabets around the world. Some countries have similar manual alphabets with a few modifications, borrowing an alphabet from one another, but their signed languages remain distinct.