What does '잘했어' mean in Korean?
Is this expression informal and when can I use it.

Question

Answer

잘 - well
하다 - to
했어 - did
So literally "(you) did well". More naturally "Good job!"
Yes, the speech level is informal and impolite. This form is called "반말".
You would use it with those younger or of lower status than you.
You could use it when you want to tell someone they did well^^

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You did well, good job! -Korean dictionary--> http://endic.naver.com/search.nhn?sLn=en&searchOption;=all&query;=%EC%9E%98%ED%96%88%EC%96%B4

Answer

I think the word 'great!' it's also same meaning.

Ex) y- i win the prize to competition!
      x- Great!