The one-handed American Sign Language alphabet is a set of 26 manual alphabetical letters, corresponding to the English alphabet. It is used to fingerspell a string of the alphabetical letters of a certain English word, person's name, etc.
There are different manual alphabets around the world. The American manual alphabet with a few modifications is derived from the French manual alphabet of the 18th century. Its cognates can be found in other signed languages' manual alphabets. For example, the German and American manual alphabets are similar; however, ASL and German Sign Language are completely different. For tips, see the bottom.
You can download the poster (pdf) of American Manual Alphabet.
Those images show what you're seeing as a listener. As a right-handed signer, you'd see the back of those images.
Be aware of the common confusion between F and D. Pay attention to D and F when you spell one of them.
Pay attention to the thumb for A and S.
Make sure your E is clear, keeping the thumb away from the index finger to avoid slipping it into O.
K and P have different upward/downward palm orientation.
Pay attention to the thumb for M, N, and T.