Explain the Alphabet and String

A finite set of symbols is known as alphabet. An alphabet is frequently denoted by sigma, yet can be given any name.

B = {0, 1} says B is an alphabet of two symbols, 0 and 1.

C = {a, b, c} says C is an alphabet of three symbols, a, b and c.

Occasionally space and comma are in an alphabet whereas other times they are meta symbols employed for descriptions. A language is determined over an alphabet. For instance binary language is defined over alphabet B. 

A finite sequence of symbols from an alphabet is known as string or word.

01110 and 111 are strings from the alphabet B above.

aaabccc and b are strings from the alphabet C above.

A null string is a string along with no symbols, generally denoted by epsilon has zero length.