Bodybuilding is the use of progressive resistance exercise to control and develop one's musculature for aesthetic purposes. An individual who engages in this activity is referred to as a bodybuilder.
In bodybuilding (especially in recent years), there are two different competing philosophies as it pertains to where bodybuilding should go. The first is what's often referred to as "classic" or "golden era", which is a throwback to the golden era of bodybuilding (which contains individuals like Sergio Oliva and Arnold Schwarzeneggar) where, while size is important, it's not as important as having a very visually pleasing figure which can be attained by a variety of factors; the right genetics, the proper diet, the proper exercise routine, and, yes, drugs (though there are some who are referred to as "all-natural", meaning they forgo any drug usage and instead maintain a strict diet and exercise routine without having to rely on steroids or human growth hormones. This philosophy has gotten so popular in recent years that the IFBB (the biggest governing body in bodybuilding) have set up a specific division for this bodybuilding philosophy called "Classic Physique". Adherents to this philosophy include individuals like Breon Ansley and Chris Bumstead.
The second philosophy is often referred to as "the mass monster", where it's about trying to get to be as freakishly big as possible, regardless if your figure is aesthetically pleasing or not (though there are some in this particular philosophy who do have at least mildly aesthetically pleasing bodies) who compete in what's known as "Men's Open". Popular examples of bodybuilders who follow this philosophy include Dorian Yates, Phil Heath, Ronnie Coleman, Jay Cutler, Kai Greene, and Mamdouh "Big Ramy" Elssbiay.
On top of these primary two philosophy, there are other forms of bodybuilding, including Men's Physique (usually the smaller of the bunch and not nearly as intensive as Classic Physique and Men's Open, though you still have to put in the work to maintain an aesthetically pleasing figure), 212 (which is the next step up from Men's Physique where you have to maintain a weight of at least 212 pounds to be eligible), as well as women's bodybuilding, which can be split up into Bikini, Figure, Fitness, and finally Women's Bodybuilding.
In the gym, most Bodybuilders will tend to wear regular, loose-fighting gym clothing to allow for freedom of movement as they do their reps to build up their figures (though in the case of female bodybuilders, it's the usual skin-tight clothing most female gymgoers tend to wear).
On the stage, however, Bodybuilders will generally wear the least amount of clothing possible in an attempt to showcase every aspect of their physiques in an attempt to win the competition. They will also put on a lot of spray tan in an attempt to highlight their muscles even more and make them pop under the stage lighting. The main exception to this are those who compete in Men's Physique, who often wear regular boardshorts most would see on swimmers and surfers when they compete on stage.