Tell them apart
Some ideas are clearest side by side. These entries take two concepts people routinely confuse - and let you feel the difference in an interactive demo, rather than just reading two definitions and hoping the distinction sticks.
An affordance is what an element lets you do - a button can be pressed, a slider can be dragged. A signifier is the cue that tells you so - the shadow, the label, the cursor change. The affordance is the possibility; the signifier is the advertisement for it.
Two quick, cheap tests that answer different questions. The five-second test shows a screen briefly and asks what people took away - does it communicate? The first-click test asks where they'd click to do a task and watches - can they find the way? Impression versus action.
Both are about "how many", but they measure different things. Hick's law is about choosing - decision time grows with the number of options. Miller's law is about holding - working memory keeps only about seven chunks at once. One is speed of choice; the other is capacity of memory.
Two ways to tell someone something happened. A modal stops everything and demands a response before you can carry on; a toast slides in, says its piece and leaves on its own. One interrupts, one whispers - and using the wrong one is its own usability problem.
Two ways to handle a list too long for one screen: pagination breaks it into numbered pages you step through, while infinite scroll loads more automatically as you reach the bottom. One gives you control and an end; the other gives you flow and no edges.
Recognising something you're shown is far easier than recalling it from a blank memory. Good interfaces show the options instead of making you remember them.
Two routes to "you can trust this". Social proof points sideways - lots of people like you chose this. Authority points up - an expert or credential vouches for it. The crowd versus the expert; both borrow credibility, from very different places.
Both hide sections of content behind labels you click to reveal - but tabs lay the labels in a row and show one panel at a time, while an accordion stacks them vertically and lets panels open in place. The choice is mostly about how many sections there are and how wide the screen is.
Two ways to find usability problems. Usability testing watches real people attempt tasks and surfaces what actually trips them up. Heuristic evaluation has experts inspect the interface against known principles. One reveals reality; the other predicts it - and they catch strikingly different things.
UI is the surface - the visible, interactive layer you look at and touch: colours, type, buttons, layout. UX is the whole experience of using the thing, screens and everything around them: finding it, the flow, the wait, the recovery when it goes wrong. UI is part of UX, not a synonym for it.
A definition tells you what something is; a comparison tells you what it isn't, which is often the part that actually trips people up. Knowing that a modal interrupts while a toast whispers, or that UI is the surface while UX is the whole journey, is what stops the two getting muddled in a meeting.
Want the rest of the collection? Read any term in the UX glossary, drill them with flashcards, or check yourself on the UX quiz.