As conjunctions, they are used interchangeably in the past tense. However, the general rule is that "while" is used with progressive tenses (ongoing/continuous action) and "when" is used with simple past tense (occurred only in the past). - "While it was raining..." vs "When it rained..." - "While I was watching TV..." vs "When I watched TV..." - "While the phone was ringing.." vs "When the phone rang..."