While to introduce a time clause: 

While can be used in a number of different ways. We use it, first and foremost, when we want to talk about things that happen simultaneously. In this sense, it is similar to ‘as’ and ‘when’. All of these conjunctions can serve to introduce a longer background situation which started before the shorter action. Consider the following and, at the same time, note the use that is made of the past continuous in these contexts.

'I completed the crossword as I was talking on the phone.' 

'I remembered that I had a letter to post when I was walking past the post box.' 

'While I was reading the newspaper, my wife was ironing my shirts.' 
As you can see from the above examples, while is particularly useful if we are discussing long actions and wish to draw attention to the duration of the activities. Consider the following: 
'I’ll prepare breakfast while you’re having a shower.' 

'While I was recovering in hospital, my wife was enjoying a holiday in Cyprus.' 
Note that if the subject is the same in both clauses, a participial construction may be used, particularly in written English. Compare the following: 
'She completed her first novel while working for the local newspaper.' 

'She completed her first novel while she was working for the local newspaper.' 



while / whereas to link two ideas that contrast with each other:

Note that while does not always refer to time. It is also used to balance two ideas that contrast with, but do not contradict, each other. In this sense, it is similar to whereas. Consider the following: 

'While I like all types of fish, my girlfriend always chooses meat dishes when we go out to eat.' 

'Some married couples argue all time, whereas others never do.' 

'We would always choose somewhere in the mountains for a holiday, while our children always want the seaside.' 
Note that whilst we would use while or whereas within sentences to contrast two ideas, across sentences we would need to use ‘however’ or ‘on the other hand’. Compare the following: 
'In the UK the hottest month of the year is usually July, whereas in southern Europe the hottest period is usually in August.' 

'In the UK the hottest month of the year is usually July. On the other hand, in southern Europe the hottest period is usually in August.' 

'Britain secured only one gold medal in Atlanta four years ago, while at Sydney 2000 we ended up with eleven.' 

'Britain secured only one gold medal in Atlanta four years ago. At Sydney 2000, however, we ended up with eleven. 

As the test is difficult, you had better get some sleep. 
'As' means the same as because. 'As' tends to be used in more formal, written English. 
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