What is "Focus"

I'm new to multiboxing but a veteran of WoW, recently coming back after a 6 month leave.

I keep reading about this magical thing called "Focus" as part of macro's but I'm not really sure what it means or what it does...

Can someone give me a detailed explanation of what it is, how it works, how it's different than "target", and how it is used?

http://www.wowwiki.com/HOWTO:_Make_a_Macro#Using_Focus

You can think of focus as a second target, essentially, with some minor differences in behavior.