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What is the difference between a Framework, a Toolkit and a Library?
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A Framework enforces you write code in its term. Frameworks call your annotated or marked code. A toolkit is a set of functions. Your code calls functions of a toolkit.
Libraries provide developers with predefined functions and classes to make their work easier and boost the development process. Framework, on the other hand, is like the foundation upon which developers build applications for specific platforms.
The technical difference between a framework and library lies in a term called inversion of control. When you use a library, you are in charge of the flow of the application. You are choosing when and where to call the library. When you use a framework, the framework is in charge of the flow.
When you're trying to define frameworks in the context of JavaScript framework vs library, think of it this way: JavaScript libraries are like pieces of furniture that add style and function to an already constructed house. Frameworks, on the other hand, are a template you use to build the house itself.
The most important difference, and in fact the defining difference between a library and a framework is Inversion of Control.
What does this mean? Well, it means that when you call a library, you are in control. But with a framework, the control is inverted: the framework calls you. (This is called the Hollywood Principle: Don't call Us, We'll call You.) This is pretty much the definition of a framework. If it doesn't have Inversion of Control, it's not a framework. (I'm looking at you, .NET!)
Basically, all the control flow is already in the framework, and there's just a bunch of predefined white spots that you can fill out with your code.
A library on the other hand is a collection of functionality that you can call.
I don't know if the term toolkit is really well defined. Just the word "kit" seems to suggest some kind of modularity, i.e. a set of independent libraries that you can pick and choose from. What, then, makes a toolkit different from just a bunch of independent libraries? Integration: if you just have a bunch of independent libraries, there is no guarantee that they will work well together, whereas the libraries in a toolkit have been designed to work well together – you just don't have to use all of them.
But that's really just my interpretation of the term. Unlike library and framework, which are well-defined, I don't think that there is a widely accepted definition of toolkit.
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Martin Fowler discusses the difference between a library and a framework in his article on Inversion of Control:
Inversion of Control is a key part of what makes a framework different to a library. A library is essentially a set of functions that you can call, these days usually organized into classes. Each call does some work and returns control to the client.
A framework embodies some abstract design, with more behavior built in. In order to use it you need to insert your behavior into various places in the framework either by subclassing or by plugging in your own classes. The framework's code then calls your code at these points.
To summarize: your code calls a library but a framework calls your code.
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