What is the Difference between Thunderbolt-3 and USB-C?
Short Version
Thunderbolt 3 and USB-C share the same connector, that's why they look the same. But internally they use different software protocols. Thunderbolt 3 is much more powerful, as it can transfer a lot more data than USB-C, but it needs to be licensed from Intel (and not everyone manufacturer wants to pay Intel the fees).
"Longer" Version
Technically, there is no USB-C protocol, but only a USB-C connector and devices using the USB-C connector can use all kinds of version of the USB-protocol. Nowadays we strongly recommend to buy a device which uses the USB 3.1 Gen2 protocol to ensure that you have enough bandwith.
Speaking of bandwidth: Thunderbolt 3 is capable of up to 40 GBit/s, whereas the most powerful USB 3.1 Gen2 can deliver 10GBit/s. This is also one of the reasons which no USB-C offers proper support for 4K displays with 60HZ refresh rate - these displays simply need too much bandwith than USB 3.1 Gen2 can offer.
Which Computers Include Thunderbolt 3?
All Apple laptops have Thunderbolt 3 ports built in. The exact amount depends on the device (usually the more expensive the more ports). Apart from Apple laptops there are a few other laptops which offer Thunderbolt 3 ports:
Outlook
USB4 is on the horizon which will match the speed of Thunderbolt 3, so we will hopefully soon have a world where everyone lives peacefully using the same connector and protocol.