Collateral genealogy is the study and/or research of those relatives that you are not directly related to but that could hold the key to information you are seeking regarding those direct ancestors. Some people find that researching ONLY their direct ancestors/relatives is enough for them but they are missing the possibility of knocking down brick walls if they don't research the siblings of their great-grandparents.
When I write up a family history, I want it to contain as much information as I can possibly find on that particular surname. Granted most information is usually gleaned from other databases posted online at various websites but if you take what you find and seek out more information that corroborates what you initially find then you can at least add to that information and even correct errors you find.
And sometimes when researching those siblings or collateral relatives you come across connections that you might've suspected but didn't have any evidence to substantiate that your hunch was right. Speaking of hunches, detectives sometimes rely on a 'hunch' or 'feeling' and genealogists sometimes have to do the same because its the only thing they have. If you think you know that a relative is linked to a surname you've already been researching 9 times out of 10 you are probably right.
Everytime I come across a surname that I recognize I go through my files and look for that name and then see if I can connect the dots and find a common ancestor that they share. You'd be surprised how many families have intermarried in different generations, especially if they both lived in small towns. Some of my Ohio ancestors have done this more than once and my instincts usually tell me there's a connection somewhere but it always takes more research to hunt it down and find it.
Never brush off the possibility that your great-grandfather's brother is the key to finding out your great-grandmother's maiden name, it just might surprise you to learn that the two brothers married two sisters and not only her maiden name but her parents are waiting to be discovered. Give collateral genealogy research a try.