A brief list of the metaphysics of Aristotle and Aquinas. This is the philosophy of changing things; what specific changing thing there are, and what their nature is, is the job of science. This is much more general than natural science, applying to changing things in general, regardless of what types of changing things exist.
Form and Matter
Typical objects consist of matter that is structured a certain way. For example, an elephant consists of matter that is structured in the form of an elephant.This also entails...
...that they have a disposition to behave a certain way. Example: an elephant is disposed to reproduce, run away from enemies, eat, and so forth. This also entails...
Actual and Potential
...that a behavior is a potential to act a certain way, whereas that object is actually acting a certain way now. Example: right now the elephant is actually sleeping, but potentially running away from a hunter; later, it will be actually running away from a hunter.
Existence and Essence
From the "form" above, we have the definition of something, such as "an elephant." If that essence/form is to be real, it must be conjoined with matter. But for most things, this definition does not entail that thing's existence. Example: what an elephant is (large mammal with trunk, ears, etc) does not entail that it exists. It may or may not exist, but its definition alone does not entail that it does.