Images, sounds, animations, word processing files, etc.
The first encoding and decoding process for doing this was uuencode and uudecode (Unix-to-Unix encode and decode)
This was usually done "manually" (before modern email clients)
But, there are many variations of uuencode and uudecode
Modern email clients use:
MIME (Multipurpose Internet Mail Extensions) (also known as Base64) -- designed as replacement for uuencode and uudecode
BinHex -- used almost exclusively on Macintoshes