The three words get used loosely, and the shop is usually the one choosing which word to use. What separates them shows up in the process, so start there.
Off the rack
A suit made to a size, then altered to you. Sizes are averages. A 40R exists because enough chests sit near 40 and enough backs near that length, not because yours does. Alterations will fix the sleeve, the waist and the trouser. They will not move the shoulder or change how the chest sits, because those were settled in a factory before anyone saw you.
Made to measure
A house pattern adjusted to your numbers. Someone measures you, picks the closest base pattern and changes it: length here, a little more room there. The fit starts far ahead of a size. The pattern still started as somebody else's.
Bespoke
The pattern starts at you. Your measurements, your posture, the shoulder that sits lower than the other one, and then the garment is corrected on your body before it is finished.
Three questions
The labels move around, so ask about the process. Were you measured in person, by the people who answer for the result? Is there a fitting on the actual garment before you take it home? Do your measurements stay on file, so the second suit starts where the first one ended?
Any shop can answer those three in a sentence, and the answers tell you more than the word on the window.
Here
Two fittings. The first takes the measurements, the second happens on the finished garment, and each one is described here. The measurements stay on file, so the next jacket, shirt or pair of trousers is cut to the same ones. The fabric comes from Italian mills, and the wardrobe runs past suits to shirts, polos, denim, shoes and travel blazers.
If you are weighing the three, the fastest way to know what you are being offered is to sit down and be measured. That is what a first fitting is.