Think of a mobile hotspot as a wireless modem, only instead of the service coming through your home phone line it comes via a cellular connection. It will allow you to connect to the Internet via WiFi where ever you can get a cell connection. So yes, if you have a good cell connection, you should be able to use it in the car.
If you have a smart phone, it may already be able to act as a hotspot and you don't need to buy anything else. I have only a basic cell phone though and because I had no interest in changing that, so I had to buy a separate hotspot.
The hotspot I got is only 3G and runs on the Sprint network (no roaming) but that is perfectly fine for my needs. I like it because I only have to pay for my actual usage, which should be low as long as I pay attention to what I'm doing (i.e. don't download movies or software updates via the hotspot, wait until I'm home on the standard WiFi, etc.). There are 4G ones out there, but they usually involve a monthly fee, although with many of the pay as you go ones you can choose to have service one month, and not the next.