[otikik]

There's 3 kinds of dependencies. The best way to describe them that I have found is a metaphor with the Terminator movies.

* Libraries are tools that your app uses. For example, the JSON parsing library. They are relatively easy to switch from, as they are (usually) used for very specific tasks. For the terminator, a library would be a shotgun.

* Frameworks are pieces of software your app embeds into. They have a set of conventions and procedures that condition the shape of your app. In exchange, they provide a lot of baked-in functionality, which is usable by your app right away. Ruby on Rails is an example of a framework for web development. They are more difficult to switch from, because your code is "shaped" in a certain way and it relies on the framework to several tasks for them. For the terminator, a framework would be a motorbike. It's difficult to jump from a running motorbike to motorbike, if there's a T1000 chasing you on a truck (that would be a deadline). It can be done, though. Much easier to do if you prepare in advance.

* Game engines are similar to frameworks, except the dependency goes deeper. Your game is not embedded into the engine. Instead, it is made of the engine. Unity is a game engine. It's very difficult to switch from one game engine to another one. The engine would be the terminator's endoskeleton - the metal skull, torso, arms and legs, plus the initial "bios". Your game would be the living tissue put on top of that metallic frame, as well as the directives programmed in the brain ("Protect John Connor").