It's just a way of naming the technology - the G stands for 'generation'.
First we had standard mobile phones, then digital mobile phones that could text and send SMS.
Then we had the third generation, 3G, that could connect a smart phone to the internet so that people could access their email, facebook, browse the internet on the move.
The current standard is 4G, which is a much quicker version of 3G and allows users to stream tv (theoretically) and makes the internet much faster and smoother.
5G is already being developed.
So, your device - smartphone, tablet,etc - will need to be 3G or 4G enabled and you will need a data plan either as contract or pay as you go to use it to connect to the internet without wifi.
I use my tablet as a satnav and it works very well. The app I use doesn't need the internet and uses the same system as satnavs, so it doesn't use any data for that job.
I would strongly recommend you get a 4G device and data plan. Not all providers supply 4G, such as Virgin Mobile.