Don't call us...

A library is a collection of functions called by your program. A framework is a large, monolithic program into which you add your code. Your code calls a library. A framework calls your code. Hence the expression:

Don’t call us. We’ll call you.

Your software can use multiple libraries, even libraries that do similar things. In contrast there can be only one framework at a time: if you settle on Django, you can’t use Flask. In the real world, hobbies are like libraries: you can have several. In contrast religions and ideologies are frameworks: you can have only one. A Bushido martial arts framework can coexist with an Independent Freewill Baptist framework in the same person only if you create a subclass from the Bushido and Baptist classes.

Careful when choosing a framework—replacing it is hard. And you’ll find that many frameworks aren’t necessary if you understand the first principles of the domain in which you work. The more you know the less you need a framework.