1/2" is abt 12mm and 3/4" is abt 18mm. Valves got few types, commonly used is ball valves, gate valves and globe valves. If what you are looking at is those that are used domestically, a ball valve will do, its easier to use. Pipes and tubing can say they are same, can also same time say not the same, some people call hoses as tubings also.
So if you can be alittle more specific, like where and what purpose usage, maybe i can help you abit.