Basically, you can make an image show much more detail or properly expose every aspect of the image. This effect is not possible in 1 exposure because of the nature of cameras.
The bad: A lot of people try to imitate this look and even label their extremely overdone edited photos as HDR. When in fact they are nowhere near an HDR image. Most of the time, they contain less dynamic range than even the original single image.
Pro: That's not HDR, that looks like pure crap.
Photo Noob: You don't know nothin' yo. This is tyte. I'm more a pro than you are.
fauxtogropher: this is my really great hdr photograph, look how bright and blinding it is, doesn't it just give you the best headache? I love using overdone styles in my photography and botching them then tricking people into thinking that I have photographic skill.
Photographer: Poor man, doesn't even know how to take a decent unedited picture. *shakes head and walks away*