A field refers to an individual piece of data within a record of a file.  The term field is also used when referring to individual pieces of data shown on a CU*BASE screen. 

If you think of a file as a table, then put another way, a column holds an individual piece of data within a row of that table. For example, the MASTER table, which contains membership information, contains a column for first name, a column for last name, one for city, one for work phone, etc.