Basically, throttling is the slowing down of your data speeds. Nowadays, when you see cell phone plans with data allowances (data limits) it typically doesn't mean that after reaching those limits, you cannot use data at all—or else like in the old days get charged an extreme amount of money if you do. Instead, reaching the limits of your limited data plan typically means your data speed will be slowed down (throttled) until your next billing cycle.
Data Prioritization Defined
When you look closely at the details/fine print of unlimited data plans, you are very likely to find language informing you that after so many gigabytes of data usage within a billing cycle, your speeds may be slowed down during times of heavy network congestion. This is due to deprioritization which essentially means that at times your access to high-speed data may be thwarted in favor of others. Who are these other people that get to keep using high-speed data? Generally, they are people who haven't reached that data deprioritization threshold. In some cases they may even be people who pay a little extra money to not be deprioritized as early as most people.
Throttling vs Deprioritization: What's the Difference?
It might be hard to look past the basic theme of throttling and deprioritization—both potentially mean your data can be slowed down. So, is there really a difference between the two? And does it even really matter? The difference between throttling and deprioritization is important and could play a role in determining the plan you buy. This is especially true if you are a heavy data user as you are more likely to encounter either one. The big difference between throttling and deprioritization is how each relates to network congestion. On one hand, throttling has nothing to do with congestion. With certain plans, once you reach the cap you can expect your data to slow. On the other hand, deprioritization only occurs (theoretically) in periods of heavy network congestion.
A Simple Analogy
Throttling: You go to a buffet (the mobile network) and are handed a plate (your data plan) and are told you pile as much food onto the plate as you want, but may only go to the buffet line once.
Prioritization: You go to an all you can eat buffet (the mobile network) and can go to the buffet line as much as you want, but if you have already filled your plate once and there are others in line that have not had a chance to eat yet, you will be asked to go to the back of the line and wait to go through again.