When one explores an active building the focus is usually on socially engineering, avoiding authority or seeming inoccuous enough to be ignored. One uses their wits and stealth to explore the off-limits, secret, and often forgotten parts of active buildings without being apprehended, out of the basic curious instinct of wanting to know what's behind every door.
Abandoned buildings are entirely different -- they are far more dangerous, more decrepit, and prone to a different sort of occupancy altogether. There you worry not so much about being approached by someone who wants to know if they can "help you" but rather being told on no uncertain terms by permanent residents that you are unwelcome. You bring light sources, you explore everything, you see as much as you can. Abandoned buildings contain stories of the utility they once held and can be truly fascinating sites that bear hours upon hours of exploration. That said, they are inherently unsafe -- many have holes in floors, long drops, weak staircases, and other such horrors. A good abandoned building is a fantastic thing to find but once inside, you should take nothing for granted -- especially your safety. It's easy to become comfortable with a hostile environment, but they have a tendency to surprise you.
The exploration of buildings is driven by the need to know and the urge to make forbidden space your own -- to become as familiar with your urban surroundings as possible. In an abandoned building, you can be in the very downtown core of a busy city at rush hour and still be the only person in the entire structure. Some of these places are the most private places on Earth, and the sensation of being there is indescribable. The basic need to know what's on the floors a hotel elevator won't stop at and what's behind the barbed wire surrounding an ancient industrial monument is what compels us. As always, the risks can be high, so stay safe -- or, if you can't take responsibility for yourself, stay home.