The main difference between the two is probably the desktop environment. The Ubuntu desktop can be very helpful when you are using a desktop or your own laptop. The Ubuntu server will work the way that it should which is to become a server.
One of the things that you can check to see which one you are using is the presence of the GUI. This is installed in the desktop version but not the server version. The desktop version also comes with a server and this is nonexistent with the server version. The question of which one you are going to use will depend on what exactly you are looking for and what you need to use.
Ubuntu is a popular type of Linux installation. There are two different types of types, including desktop and server types. If you have the desktop version, then you usually will use the internet for word documents, surfing the internet and possibly playing the internet or computer games. If you need more complicated things, then you can use the server version in order to create and update web pages and host files.
If you have the desktop version, then it comes already installed on the computer. However, the web version does not come already onto a computer. The server version does not have a lot of applications already installed on it whereas the desktop already has many different applications installed on it.