When a small kitchen runs out of cabinet, the next question is usually "build more shelf, or add a cart?" They solve different problems, and picking wrong means money spent on storage you still don't use. Here's the actual difference, so you buy the one your kitchen is actually missing instead of the one that looks useful in a photo.
Cabinet shelves: fixed, hidden, free of floor space
Adding a shelf inside a cabinet is cheap and uses no floor. Stackable bins ($17) do this job — they turn one shallow shelf into two or three layers you can see into. The downside is reach: a bin in the back of a deep cabinet still needs unloading to get to, which is why people stop using the back half.
Slim cart: movable, visible, uses the gap
A slim rolling cart ($40) lives in floor space a cabinet can't — the gap between appliances. It rolls out, so the back row is as easy as the front, and it moves with you to where you're cooking. The trade-off is floor footprint and a slightly smaller capacity than a full shelf, but the gap was empty anyway.
How to choose in one question
Is your problem dead cabinet air, or dead floor gap? Dead air → stack bins inside the cabinet. Dead gap → roll a cart into it. Most tight kitchens have a bit of both, which is why the two work best together: bins inside for the dry goods, cart in the gap for the daily-reach oils and spices.
What not to do
Don't buy a wide cart hoping it'll fit — measure the gap first. And don't stack bins so high they tip; keep the heavy stuff low. Both failures are why people give up on small-kitchen storage and go back to the counter pile they were trying to escape in the first place.
Shop the slim cart or the stackable bins, and see the full range.