It looks like every time you go to the gym, they've got another fancy new machine. The number of ways to build exercise machines seems to be almost infinite, and coming up with new ideas for them is a lucrative industry. When you look at gym equipment, though, you can pick out some of the most useful and flexible machines and tools, and separate them from the gimmicks.
The number one piece of gym equipment, without question, has to be weights. Weights are just heavy pieces of metal, and yet are a vital ingredient of almost every gym machine, as well as very useful in their own right. Lifting weights can strengthen the majority of the muscles in your body, if you use a variety of lifting techniques.
Beyond that, many classic gym machines replicate real-world sports and activities in order to exercise your muscles. Treadmills, for example, reproduce walking and running, rowing machines allow you to train your arm muscles without actually going out on the water, and exercise bikes simulate cycling. Punching bags allow you to box without an opponent. Each one of these sports simulations has a specific reason for being included, usually that it is good for a specific muscle group, as well as being useful for people who are actually training for the sport in question.
If you're thinking of buying gym equipment for your home instead of going to the gym, weights are probably the best thing to get, as they are versatile and compact. Machines like treadmills and exercise bikes can be ten times the size of a set of weights, not to mention ten times the price, yet can not do as much for you as a simple set of weights can. Try not to get talked up in the latest 'as seen on TV' gimmick, as most advertised gym equipment will only do anything for one particular muscle group, and even then probably will not do much for that. Make sure you know exactly what you're buying and what it will do for you before you make the investment.