Sticky Session

What does the term sticky session mean in a web-farm scenario?

Sticky session refers to the feature of many commercial load balancing solutions for web-farms to route the requests for a particular session to the same physical machine that serviced the first request for that session.

Why would you use a sticky session?

This is mainly used to ensure that a in-proc session is not lost as a result of requests for a session being routed to different servers.

What is the potential disadvantage of using a sticky session? 

Requests for a user are always routed to the same machine that first served the request for that session, sticky sessions can cause uneven load distribution across servers.