When is past tense and past continuous tense used?

Past simple is used to talk about something that happened in the past and is now finished

I ate my lunch.

Past continuous is used to talk about some activity that happened over a period of time in the past:

I was watching TV.

They are often combined together to talk about an activity that happened over some time and another event that happened during that time:

While I was watching TV I atemy lunch.