A flat-file database model is any system using a single file for storage with one record per line, usually plain-text or text mixed with binary data.
A "flat file" is a plain text or mixed text and binary file which usually contains one record per line or 'physical' record (example on disc or tape). Within such a record, the single fields can be separated by delimiters, e.g. commas, or have a fixed length. In the latter case, padding may be needed to achieve this length. Extra formatting may be needed to avoid delimiter collision. There are no structural relationships between the records.
Typical examples of flat files are /etc/passwd and /etc/group on Unix-like operating systems. Another example of a flat file is a name-and-address list with the fields Name, Address, and Phone Number.
Source: Wikipedia (Flat file database)
