In the context of a shoot, Uncle Bob is usually a very affluent gent (or has enough money to spend on some prosumer or pro gear) whose photography knowledge is usually at amateur levels, whose company was asked for by the event attendees, but whose photography was not.
He has a habit of wanting to "talk shop" with the photographers who are being paid to work the event and generally, gets in the way. Sometimes he even tries to shoot over your shoulder as you pose a subject.
Uncle Bob is very eager to share his work, and to his credit, gets some keepers. But, he'll always deliver faster than you and his keepers could mean lost sales to you.
I remember one of my first gigs ever - and was using a 20D with a 50 f/1.4 and 100 f/2 to photograph a dance recital. I was the official photographer and ran the cards out of the auditorium every 5th dance or so, so that we could sell on-site. I got stopped by Uncle Bob on one of these runs; his question was why his photos were not coming out like mine (1 Series + L glass in his hands)? Classic Uncle Bob. (My response was, "I'd be more than happy to discuss after the event, but in the meantime, we'd appreciate your business if I've taken any shots that you like")
But, as photographers, I think it's important to note that we can all become Uncle Bob. We all want to shoot, we all like to shoot, and sometimes, we are the ones invited to an event where our company is wanted, but our photography is not.
So, don't be Uncle Bob.