Alpha, beta and delta are the first, second and third letters of the Greek alphabet. They are used to distinguish certain related terms in chemistry. For example, they are used to differentiate alpha and beta carbons, or delta and sigma (another Greek letter) bonds.
In organic molecules, the first carbon atom attached to a functional group is called an alpha carbon, whereas the second is called a beta carbon. Delta and sigma bonds are chemical bonds named so because of their shape. There are several other uses in math, chemistry and other sciences for these letters. For example, alpha, beta and delta radiation and particles exist as well.