[theteapot]

> Dependency injection is just passing your dependencies in as constructor arguments rather than as hidden dependencies that the class itself creates and manages.

That's the best way to think of it fundamentally. But the main implication of that which is at some point something has to know how to resolve those dependencies - i.e. they can't just be constructed and then injected from magic land. So global cradles/resolvers/containers/injectors/providers (depending on your language and framework) are also typically part and parcel of DI, and that can have some big implications on the structure of your code that some people don't like. Also you can inject functions and methods not just constructors.

[rjbwork]

That's because those containers are convenient to use. If you don't like using them, you can configure the entire application statically from your program's entry point if you prefer.