I came across this article Mediums for Oil Painters by Kenneth Freed. It gives a run down on the different components of oil painting mediums:

To painters, discussing mediums can be like a political debate. There are pro-Maroger, anti-Maroger, pro-natural resins and anti-natural resins, as well as pro-alkyd and anti-alkyd proponents. The disputes are always about potential cracking, lack of adhesion and yellowing. I have always been the curious type and have experimented with almost every medium that I could get my hands on. However, if you are new to painting, the best approach is to experiment with the paint right out of the tube so that you can understand and fully integrate into your procedure what the paint can do without additives.