It will be way quicker to just link you to the master than explain it.
http://en.wikipedia.org/wiki/Henri_Cartier-Bresson
I'm not a student of photography (or even an art critic at all), but my quick perusal of links there doesn't make it clear as to whether or not a lack of consent is integral. For instance, the link of Candid photography includes this:
Almost all successful photographers in the field of candid photography master the art of making people relax and feel at ease around the camera, they master the art of blending in at parties, of finding acceptance despite an obvious intrusive element - the camera.
I understand that such a succinct definition can't be all-encompassing, but there's almost an implied consent there. There's the sense that the skilled photographer can get natural shots for people that know they are being photographed. Mind you, this obviously isn't a requirement. The opening paragraph includes this:
Thus, the candid character of a photo is regardless of the subject's knowledge or consent as to the fact that photos are being taken, and regardless of the subject's permission for subsequent usage such as distribution, but related to the apparent absence of posing.
So obviously, candid photos include those which lack consent. But they're also making it clear that this isn't integral to the shot. Not wanting posed photos doesn't inherently mean taking a shot where there person is unaware of a picture being taken. Just that it's necessary for the person to be inconspicuous enough, or the photographer to have put the subjects at enough ease where they are not reacting to the camera's presence.
Mind you, I do understand that there are people out there that want to capture real (we can include quotations marks there optionally) moments from an event. And I don't think it's impossible for a photographer to take unobtrusive photos of patrons that didn't explicitly consent to having their photo taken, or that such photos need to be regarded as disrespectful to the subjects if they aren't consulted before or after the photo is taken.
Still, I do think that people should err on the side of soliciting permission from people that may not feel comfortable being the subject of photos. Further, I think that some candid shots can probably result in a clear consensus in regards to people finding it disrespectful. But I'm also not advocating that we necessarily need to tackle anybody with a camera that looks like they may be taking a picture of someone that isn't aware of it. The shot may be artful and respectful. But I'm not willing to give everyone with a camera pointed at a person not posing for a photo that benefit of the doubt.