If something happens while something else is happening, the two things are happening at the same time.
They were grinning and watching while one man laughed and poured beer over the head of another...
I sat on the settee to unwrap the package while he stood by...
If something happens while something else happens, the first thing happens at some point during the time that the second thing is happening.
The two ministers have yet to meet, but may do so while in New York...
Never apply water to a burn from an electric shock while the casualty is still in contact with the electric current.
You use while at the beginning of a clause to introduce information which contrasts with information in the main clause.
Marianne was tempted to turn the large rooms into traditional French-style salons, while Howard was in favour of a typically English look...
The first two services are free, while the third costs ￡35.00.
You use while, before making a statement, in order to introduce information that partly conflicts with your statement.
While the numbers of such developments are relatively small, the potential market is large...
While the modelling business is by no means easy to get into, the good model, male or female, will always be in demand...