when vs while

他們一邊等一邊談話。

When used as a conjunction, 'when' could mean 'at the time that' or 'during the time that'.
When used as a conjuction, 'while' could mean 'during the time that' but not 'at the time that'.
We should use a continuous tense after 'while' to mean 'during the time that'.Therefore, it is ok to say "Mary was watching tv when he called" but not ok to say "Mary was watching tv while he called". We can also say "He called while/when Mary was watching tv."

'They were talking while they waited' is wrong. Although "They were talking while/when they were waiting" is grammatically correct, it is better to say "They talked while/when they were waiting".

While and whilst are conjunctions whose primary meaning is "during the time that". An example is:

The days were hot while we were on vacation.
I read a magazine whilst I was waiting.
While and whilst can nowadays legitimately be used in the contrastive sense of although or whereas, provided that it is not ambiguous (although some commentators, such as Eric Partridge, have frowned upon such use):

While Sally plays, Sue works.
This sentence can mean either "During the time that Sally plays, Sue works" or "Although Sally plays, Sue works".
When may refer to:

When?, one of the Five Ws, questions used in journalism
WHEN, the former call letters of TV station WTVH in Syracuse, New York, U.S.
WHEN (AM), a sports radio station in Syracuse once affiliated with WTVH