Web garden in asp.net :
When an application is hosted by multiple processes on the same server it is said to be a web garden environment. An application pool with multiple worker process is called Web Garden, means single server and multiple CPUs.
All IIS Request process by worker process ( w3wp.exe), In web garden scenario If we use Session Mode to "in proc", our application will not work correctly because session will be handled by different Worker Process. For Avoid this Type of problem we should have to use Session Mode "out proc" and we can use "Session State Server" or "SQL-Server Session State".
Web farm means running one asp.net application on multiple machine.