join command is used to join two files based on a key field present in both the files. The input files can be separated by white space or any delimiter. join can be looked at like the join functionality in any RDBMS(database). However, the only difference being in the database join we get only the columns which we give in the select clause, in case of unix join, you get all the columns of both the files. And yes, the join can be done using only one key field, not multiple.