What different between 'focus on' and 'concentrate on'?

Your concentration is high when you are studying or reading a subject you know or fixing a motor you are familiar with.  focus is comes before understanding, you are learning and using greater energy and once you know, your concentration becomes more like familiar.

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I don't see any major difference.  focus is a metaphorical concept for concentration of energy.  The word concentrate is part of its meaning.  Focus just implies that there is something specific you are trying to concentrate, rather than just a generic concentration that would suit whatever context it is being used in.

I will try to explain the difference between concentration and attention.

Concentration: is when you force your mind in some direction. But it is not creative. It can't bring creative solutions.

Attention: You don't force the mind in some direction. You just observe your thoughts. It helps in giving creative solutions come.

They are essentially synonymous. In some contexts, “focus” can mean to narrow your attention to a very certain thing, while 

“concentrate” can mean to spend time deeply thinking about that thing.

not much difference. To me, "focus" implies a tighter inspection than "concentrate".