The UV/Image Editor is used for texturing and UV mapping. This is basically add a image or a texture to an object inside Blender.
You can do this by selecting your object in the view port and then going on edit mode (tab-see view port). Once you have done this press A to select all of the vertices and then press U. It will then come up with the following menu:
The most simple option is to select smart UV Project, this means that the computer will try to find the most simplest route unwrapping the object. Once you have done this select in the menu the UV/Image Editor. Once you have done this you should see your objects faces projected onto a single wall. This is what you would get if you did it for a cylinder:
You can now add an image of save this projection so that you can texture it. You can add an image or texture by pressing the NEW button near the bottom of the panel. Once you have pressed this you can add a name to your projection and it will add it to Blender. Next Press N and the left hand panel will appear. This is the main panel you will deal with in the UV/Image Editor. It should look like this: