If you have ever had to buy software for your PC you may well have come across the variety of licences companies like Microsoft use to sell software. Buying and installing the incorrect licence be very frustrating as getting the licences swapped may not always be as simple as just taking it back to the shop.
An OEM licence is limited to the machine and not the person. If you purchase an OEM licence of your operating system for your PC and then a year later you replace your PC you would have to buy a new operating system licence. When you buy a new PC you will often find that software is pre installed on it, in most cases it will include an OEM licence of the software.
An upgrade licence is (as the name says) an upgrade based on a full licence you already have. This is often seen when Microsoft release a new version of their windows operating system. When the new operating system comes out, if you have a full version of the previous operating system you would be able to buy an upgrade for less than the price of a full licence.
The main drawback of using an upgrade licence is that it relies on the previous licence for the upgrade. If you ever need to reinstall the software you may need both licence keys, in some cases you may need to install the older software first then the upgrade.
Full licences will be the most expensive but they are also the most flexible. The licence relates to the person and not the machine. If you upgrade to a new PC you can just install the software in the same way you had done on your old machine. You may only be allowed to have the software installed on one machine at a time but you are free to move it from one machine to another as you please.