What are "functional" orthodontics?

I'd like to know the differences between a "traditional" and "functional" orthodontics? I never really knew there were two kinds. How come you don't hear very often about "functional" orthodontics?
Update: @Tooth975, thank you answering my questions. I was wondering if I could ask you more questions about this. I have a traditional orthodontist and I was wondering a few thing.
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