> are synonyms.

Did I say that?

> ...all have distinct and quite distinctly different purposes

I hit a nerve, huh. Let me just say, that it seems a lot of people are not very happy with these designations, resulting in coinages left, right and centre.

I found this info at dictionary.com:

information science -
The science that is concerned with the gathering, manipulation, classification, storage, and retrieval of recorded knowledge.

information technology -
<business, jargon> (IT) Applied computer systems - both hardware and software, and often including networking and telecommunications, usually in the context of a business or other enterprise. Often the name of the part of an enterprise that deals with all things electronic.
The term "computer science" is usually reserved for the more theoretical, academic aspects of computing, while the vaguer terms "information systems" (IS) or "information services" may include more of the human activities and non-computerised business processes like knowledge management. Others say that IT includes computer science.

Informatics -
Information science.

computer science -
the branch of engineering science that studies (with the aid of computers) computable processes and structures