PKZIP is an archiving tool originally written by the late Phil Katz, and marketed by his company PKWARE, Inc.
The first version of PKZIP appeared in 1989. It was an MS-DOS command-line tool and was distributed as shareware with a $25 registration fee. PKZIP 1 used three different compression algorithms, colourfully referred to as "shrinking", "reducing" and "imploding", choosing between them based on the characteristics of the file being compressed. Although popular at the time, files in PKZIP 1 format are now rare, and many modern unzip tools are unable to handle the PKZIP 1 compression methods.
Some years later, PKWARE brought out PKZIP 2. This new version dispensed with the miscellaneous compression methods of PKZIP 1 and replaced them with a single new compression method which Katz called "deflating". The resulting file format has since become ubiquitous on Microsoft Windows and on the Internet - almost all files with the .zip extension are in PKZIP 2 format, and utilities to read and write these files are available on all common platforms (see zip (file format)).
PKWARE has continued to develop PKZIP, but the newer versions of the PKZIP format involve proprietry compression techniques and have never become widely used.
See also: WinZip