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^^

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

I think the word 'great!' it's also same meaning. Ex) y- i win the prize to competition! x- Great!