I was swimming while he was playing football. - These two things were happening at the same time. When I was driving to work, I met your brother. - To show an interruption of an action, or a finished event that happened at the same time as you driving. ex. "I was driving to work when I spilled my coffee." She was sleeping when I wrote a letter. - This is used to show you were doing something while I completed an action. If you used "while" here, you would say, "She was sleeping while I was writing a letter."

'when' works in all these cases, but while is fine for the first 1 too. When is a more casual word and is more common in everyday speech. Let me give an example: "I usually get some reading done when/while my wife's at work." Technically 'while' is perfect here, since the actions are at the same time, but in everyday speech native speakers will usually use 'when' in these cases.