There are two general classifications of barbell weight plates: standard weights and Olympic weights. The difference between these two types of weights is size: the Olympic weights are fitted with a larger hole, meaning a larger barbell can be used in conjunction with the weights. This ability translates into the lifter's ability to load more weight onto the bar, thereby getting a more strenuous workout. Standard barbell weight systems are smaller, and less weight can be loaded onto the barbell. Both systems have advantages and disadvantages, and choosing the best set depends largely on your fitness goals as well as your current fitness level.
Beginners who are not sure whether lifting weights will be a long-term hobby or activity will probably want to consider standard barbell weight sets. These sets are less expensive than Olympic sets, and while you will be limited in your workout capabilities, beginners are less likely to need the amount of weight made possible by Olympic sets. The weights themselves are often much smaller than Olympic weight plates, meaning storage may be easier with a standard set, and the overall cost of a standard set will be lower than that of an Olympic set.
People who are certain they will be lifting for many years, and advanced users who know they will need the extra weight, will want to consider investing in an Olympic barbell weight set. That way, you will not be limited by the weight set's limitations, and you can advance into heavier weight sets as you build muscle and better technique. The Olympic set will cost more, but the investment will pay off in the long run, especially since you will not need to buy an entirely new barbell and new weights, as will people who start with a standard weight set and want to upgrade to an Olympic set.
When choosing a barbell weight set, regardless of whether you choose standard or Olympic, you will need to make sure the set comes with enough weights to be practical. Buy at least two plates of each weight so you can even out the barbell properly, and make sure the set comes with the proper clamps or security systems that ensure the weights will not slide off the bar during use, potentially leading to injury to the lifter or damage to the weights or the floor beneath the weights. Buying a variety of weights will give you greater flexibility in your workout options as well.