What is the difference between Web farm and Web Garden?

Web Garden is the web hosting system which comprises of multiple “processes”.
Web Farm is the web hosting system which comprises of multiple “computers”.

Taken from here

A web farm is a group of two or more servers used to host the same site. Web farms increase the capacity of a web site, and improve availability by providing fail-over. Web farms are universally used for high-traffic and mission critical web sites.

When an application is hosted by multiple processes on the same server it is said to be a web garden environment.