Base-64 encoding is a character encoding scheme that represents binary data in an ASCII string format by translating it into a radix-64 representation. The base-64 term derives from its use as an early method of encoding computer files for transfer over the then-limited capacity of the pre-WWII analog telephone system (1940s), which used 64 symbols, or "bits" (therefore, "base").
The only difference between the Base64 encoding and similar encoding methods, such as quoted-printable and MIME (RFC 2045), is that Base64 encodes 64 bits at a time instead of 8 bits at a time. The use of 64 bits allows for more data to be encoded than can be done with just 8 bits, which could limit how much information can be put in an email because 8-bit email headers could only contain 20 or 30 characters.
Base-64 encoding is used for data transmission, storage and replication. It can be used as a method to reduce the size of the data as only as much as possible is encoded; not every byte needs to be encoded.