Define ABC as an abbreviation
type: ABC
C-x aig
type the text that ABC gets expanded to
use C-q C-j to enter a carriage return

Save abbreviations to a file
M-x write-abbrev-file [RET] FILENAME [RET]
The default file is ~/.abbrev-defs

To read an abbreviations file
M-x  read-abbrev-file [RET] FILENAME [RET]