Web Farm means hosting environment consisting of multiple servers which is typically connected with Load-Balancer.
It is very common to use Load-Balancer these days for high performance web applications. I encourage you to read my other blog
Load Balancer.
To know about Web Garden it is necessary for you to know Worker Process and App Pool.
App Pool is a logical grouping of applications (web) executes in a single process.To execute each Web Application in a separate process, you are required to create an App Pool for each application. App Pool enables us to isolate our Web Application for better security, reliability and availability. The worker process serves as the process boundary that separates each Application Pool so that when a worker process or application is having an issue or recycles, other applications or worker processes are not affected.
An App Pool with multiple worker processes is called a Web Garden. Many worker processes with the same App Pool can sometimes provide better throughput performance and application response time.