Yeah, margarine is definitely not just one thing -- it can vary a lot! As an example, there are only three completely vegan brands of margarine that I really know about: Smart Balance Light, Earth Balance, and Nucoa. Smart Balance Light has a lot of water in it -- it becomes obvious right away that you just have a spread rather than anything to cook with, which is how I suspect most "light" margarines are.
Earth Balance can be used both as a spread and for cooking. I think one of the major differences between sticks and the tub is the water content. Earth Balance uses these vegetable oils as a mixture: soybean palm fruit, canola and olive.
Nucoa is probably the richest tasting vegan margarine and I've only ever had it in stick form. However, it is not as healthy as Earth Balance is. The vegetable oils it uses are Liquid Soybean Oil, Partially Hydrogenated Soybean Oil.
Butter, on the other hand, while varying in quality, is just that: milk fat that's been separated from milk.