There are distinct differences between professional lenses and amateur lenses. First is obviously the price. Aside from the price, what can you expect when purchasing a pro lens over a consumer rated or "amateur" lens? The key difference, and the most important one, is the light gathering capability. Pro lenses generally gather more light. This can amount to anywhere between 1 and 3 stops of light. If this doesn't sound like a lot, consider that 1 stop equals 2x the amount of light, 2 stops is 4x and 3 stops is 8x. That's a lot of light. The way that these lenses gather more light is through the use of sharper glass and also more glass, making the professional lens significantly larger than an amateur lens, most of the time.
We also find that pro lenses are generally constructed to be more rugged so as to withstand the day to day demands of a professional photographer. As a result they have more metal parts and are heavier as well.
The question that is often asked is whether or not it's worth the expense for the hobbyist. The answer depends on how much and how you use the camera equipment. It also depends on what you are doing with the images after the image is taken. If you plan to make very big enlargements or are under low light often. This would make a big difference. If not, then you would not feel the impact of moving to a pro lens. Also, if you are going to be using your equipment a lot then you will want something that will take the abuse. Short of this it's likely best to stick with the consumer rated lens and save not only your pocket book but your back.