Every character has a numeric code called its ASCII value. A is 65, B is 66, a is 97, 0 is 48, and space is 32. These numbers are standardized. You can convert between char and int: int code = "A"; gives you 65.
And char letter = 65; gives you A. The types are interchangeable. Lowercase letters are 32 positions after uppercase. a is 97, A is 65. You can convert case by adding or subtracting 32 from the ASCII value.