What is the difference between Perfekt tense and Präteritum?

'Präteritum' is the way of telling a story. Therefore, it is also used when writing a story. A story in that sense is anything which is remote. It may be in the past, it may be in the future, it may just not be of primary concern for the person who tells it. For example "Ich spielte Fußball". Perfekt is the way of discussing something which happened in the past and is of primary and immediate relevance for the person who tells it. So, when a friend asks, why you look so exhausted and whether any of the events the other day are related to that, you may very well use Perfekt tense because it is not remote to you or him/her. For example, "Ich habe Fußball gespielt"