The Core 4
Core 4 Definitions
Plugins: are a small usable functions. They should mainly do 1 thing and be usable across a wide spectrum of Frameworks, Libraries, and Services. If I can't make a plugin work within several minutes, then I move on.
Libraries: are groups of utilities. Examples of projects I consider libraries are jQuery and LoDash. Libraries create ways for their users to more easily do things like manipulate html. I find I change Libraries minimally so I tend to go for rock solid products that are stable and well documented.
Frameworks: are projects like Angular, Ember, and React. They establish patterns for writing entire systems of code. Frameworks are the cornerstone of products I build so when choosing a framework I build little apps and think about how that Framework's functionality will apply to the project I will be using it for. I invest a few days into understanding the basics of a Framework to make sure it's the right choice for a product.
Update: Every single person that has read this post thinks that I am wrong and has valid strong opinions on why. Feel free to share your thoughts below.