做 = do，make, VERB. It is usually followed by substantives or notional verbs. e.g. 做饭 = make a meal; do one's homework; do some sports, etc. ------ 作 It is meaningful only with other characters. There are many different combinations with other characters which represents different meanings, for example: 作为 = as, prep.（I have to take exams as a student.）This combination has another meaning: achievements（= 成就 noun） 作业 = homework, noun. 作品 = works, noun（I like your works on the exhibition.） 变作/化作 = become something. You can either consider the whole word as a single verb OR “变/化” as the verbs and “作” as a preposition. 作孽 = do evil，you may find it confusing here that it means "do", but this is idiomatic. I don't know why it is "作" other than "做", maybe "孽" is too abstract? Anyway, what I am sure is that It comes from the Buddhism. People use this word with regret to bad happenings, for example, some person or thing (usually living thing) is destroyed. 作秀 = show, act, verb（act in some public situations. In most cases, this word has negative connotations, which implies that SOMEONE earns applauds by pretending themselves in the public or do some stupid things just to attract others' eyes） 作风 = working style; the way / habits in which someone usually does something, noun. 做作 = affected, adj. Interesting word as you may find it. This is also idiomatic. The two characters in the word cannot split. It also has some negative connotations which indicates that some person behaves in an unnatural or uncomfortable way. He wanted to make himself look elegant or behave elegantly and he himself though he did it, but from other people's sight of view, he is awkward and the intention is too obvious. ------- In fact they are totally different, except for the pronunciation. But they are misused by some people even native Chinese. Because of that, they are now informally interchangeable in some daily usages. But for international learners, it is recommended to distinguish them.

Hi, there some supplements of my answer. I introduced several more words and added details of usages --- some words really common but may be confusing.

http://www.italki.com/answers/question/124405.htm http://www.italki.com/answers/question/128143.htm