Base64 is a 6 bit subset of ASCII.
Base64 has 65 characters. These include all lowercase and uppercase english letters, the numbers 0 to 9, + and /. Unlike 8 bit encodings, Base64 treats 24 bits of data as the combination of 4 x 6 bit groups, rather than 3 x 8 bit groups. The character = is used to pad when needed, since the 6 bit nature of Base64 can cause there to be unused bits at the end of a byte stored in memory.
There exist other similar encodings, such as Base32 and Base16.