The most common alloy used to make alloy wheels is aluminium, but steel and even magnesium are sometimes mixed in.
Alloy wheels are made out of an alloy which is a mixture of different metals, some pure metals and some other metals with many advantages. Alloy wheels catch your attention immediately due to their look (they're shiny), but they're used because they're much lighter than steel (this lets you accelerate vehicle/ car wheels faster, stop sooner, etc...) while still having the same strength as steel wheels. This means that they offer better performance lower unsprung mass, greater strength and higher durability. If you take a close look at aftermarket wheels, you'll notice all kinds of shapes and designs on them especially in sports cars. Alloy wheel manufacturers design them to look stylish with gravity casting and are different from each other so that you can choose the one most suited wheel type to your taste. To find out whether two wheels would fit a certain car or not designers have created a standardised system called PCD (the number of bolt-holes). The PCD specifies the number and positioning of holes on a wheel hub, which corresponds to matching holes in aluminium alloy wheels.
Normal wheels are made from steel because it's very durable and easy to manufacture. Steel rims are regarded as the standard for wheel manufacturing, but alloy wheels are now becoming more popular because they offer even better performance, better traction and offer a weight reduction of the car by making lighter wheels. They need little to no repair, offer better acceleration and handling and are corrosion-free. Steel wheels have been around since the early days of cars and steel is still used today. The benefit is that they cost less than aluminium alloys. On the other hand, their weight means a slight reduction in performance. This is at least partly compensated by steel's great durability of the cast. It doesn't wear down easily so you won't need to change them as often as you would an alloy wheel.
The number of bolts on your wheel or bolt pattern describes how many bolt holes there are from one side of the wheel to the other. For example, if a wheel tyre has 5 bolts then it will have 5 bolt holes. Most alloy wheels are made up of a combination of different PCD's, for example, most cars can accommodate from 4-8 bolts on a wheel.
The main reason is to personalise your car and accentuate its style with steel wheels, but there's also an advantage in buying custom alloy rims made from magnesium: they're often lighter wheels than stock so you get better performance! They're normally thicker than steel wheels in a vehicle and made from a different alloy to normal wheels so alloy wheels tend to withstand greater stress without cracking or suffering damage.
Alloy wheels offer better performance, dissipate heat quicker and remain cooler longer. Alloy rims reduce weight on your car, improve fuel economy, efficiency, braking distances and acceleration. Alloys like magnesium alloys also wear down more slowly than normal steel rims so you don't need to replace them as frequently. They do cost more than standard steel wheels though. Besides great fuel economy there's also increased safety because alloys can withstand much greater stress of unsprung weight without suffering damage (like stacks or chips) because of high pressure die casting whereas on normal steel wheels a little bump could be the difference between a functional wheel and a bent one.
Alloy wheels last a long time as long as they're properly maintained and used under normal conditions. The last major factor is the wheel's size. Smaller wheels will wear down faster than larger ones, so if you change your wheel sizes keep that in mind.
Alloy wheels are definitely worth it if you have the extra money to spend as opposed to steel wheels. The improved performance, better fuel economy and better suspension is hard to ignore. You also get a greater range of styles as compared to steel wheels made from low pressure die casting. If your budget is tight then steel wheels with steel counterparts are still better than nothing but they aren't nearly as durable or efficient at the every level price when compared with modern alloy technology.