UX
In short: UX (user experience) describes a user’s whole experience when using a product.
In more detail: This includes understandability, speed, feel and whether the goal is reached easily.
In Depth
Approach
Research users, plan paths (user journeys), test prototypes, evaluate feedback. Appearance is only one part, see UI.
Example
A pretty form (UI) with 20 required fields is poor UX. See GUI, frontend.