That's actually a rather in depth answer, very much dependent on the load balancing solution used.
In a nutshell, you have a load balancer that takes incoming traffic and routes it to the appropriate server listed. There are a number of ways of determining where it gets routed, be it simple round robin, random, or a weighting system.
In most cases your VIP, or virtual ip, will float between two load balancers as a form of redundancy.
As I said above this is a rather generic question, and the more detail you provide, the more detailed of an answer we can give.