You could try this site http://www.vocabulary.com

It's a great source of information for learners of English where you can find a lot of articles that describe the difference between the most confusing English words. For example, you can find the answer to your question here:

http://www.vocabulary.com/dictionary/coherence

Hope it helps.

correction: ... used in the same way as 'while'.

Here's a link explaining the difference between coherence and cohesion:

http://www.bothell.washington.edu/wacc/for-students/eslhandbook/coherence

'Whereas' and 'whereby' are completely different words.

'Whereas' is a conjunction indicating a contrast, and is used in the same why as 'while'.

'English accents vary greatly across the UK, whereas they vary comparatively little across Australia.'

'Whereby' means 'by which'. For example 'There is a system whereby members can contact each other in private.'

Thank You for your help!

  Example for  WHEREAS;

    "On some days Bruce can successfully give answers, whereas on other days,  another person will offer a sufficient answer.

   "Whereas   the student wrote  several logically coherent paragraphs at the beginning of the paper,  in the latter paragraphs,  all logical coherence was lost."

    "The first efforts at English study on Italki   produced little success for the  college student, whereas the later efforts produced much success, after the student was able to see what was going on in Italki forum."

    "The student noticed that some students wrote a near perfect English,  whereas even  native speakers like Bruce  sometimes   had typing errors in their comments."