Throttling is a provider deliberately reducing internet speed once a user passes a threshold stated in the plan's terms. Rather than cutting the connection off entirely, the user remains connected but at a lower speed.

Usually still enough for basic use, such as text chat or checking a navigation map, but usually not enough for activities needing sustained high speed, such as high-definition video. The exact figure is stated in each plan's own terms.

FUP is the fair usage threshold or condition that defines when a speed reduction begins, while throttling is the action that actually happens once that threshold is passed. The two are related but not the same term.

Check data used against the plan's threshold first. If close to or past it, there is a good chance it is throttling. If still well below the threshold, it may instead be local network congestion.

No. Most unlimited plans still carry an FUP, and once usage reaches that stated threshold, throttling still applies just as it would on a regular plan. Read that plan's full terms before buying.