If you want to buy a lily pad table lamp because your desk seems a bit bare, then don't worry too much about the different lighting categories. If this is your primary purpose, then your main focus should be on the design and style of the lamp. In interior design, there are a lot of different themes. It's best that you know the theme of the room or the desk. Once you have that, then you'd want to buy a lily pad table lamp with a design that is aligned with the room's theme.
If you are buying a lily pad table lamp because you need a lighting source, then the first thing to be knowledgeable about is the difference between ambient and task lighting. These two are the major categories that encapsulate most lamp types. A lamp that is used for providing illumination to the entire room is categorized as ambient lighting. On the other hand, task lighting is a lamp that is designed to provide you with lighting to do a specific task like writing, drawing or doing paperwork.