Static screens show what an interface looks like. Interaction design decides what happens when someone uses it: what they can press or drag, how the interface responds, and how it changes afterwards. A swipe to delete, a pull to refresh and a menu that opens from the button you tapped are all interaction design.
A few principles guide most good interaction design. Affordance: it should be clear what can be interacted with. Feedback: every action should get an immediate, visible response. Predictability: the same action should give the same result. Forgiveness: mistakes should be easy to undo.
Motion is one of the main tools. A sheet that slides up from the bottom shows where it came from and where it will go. A spring that follows your finger and settles naturally makes a gesture feel direct. Used badly, motion slows people down, so interaction designers decide carefully what should move, how fast and why.
Interaction cannot be tested with static images. Designers build prototypes, from simple clickable flows to detailed prototypes with logic, to test how it feels. The challenge is that prototypes are often rebuilt by developers, and details get lost in the handoff.
Tools that store interaction as data reduce that loss. ExodeUI, which we make, keeps states, spring physics, inputs and events in one open file that runs in the real app, so the behaviour you design is the behaviour that ships.