What is a Legacy?
Legacies are students who are applying to the same university as another family member. Let's say your mom graduated from Northeastern University and you're applying there too, then you're a legacy applicant.
This is why on most college applications, including the Common Application, they'll ask about you parents education and where they graduated.
Note though that most universities have strict definitions about what other family member graduated. Your mom and dad count. Chances are your aunt and uncle don't. Sibling? Yes. Cousin? No.
Legacy Admits: The Pros
So, Dear Old Dad graduated from Virginia Tech. Since you were born, you've sported the maroon and orange, hearing Enter Sandman makes you jump, and you've planned that Ut Prosim tattoo on your right shoulder. It's the only place you want to go.
You're fairly sure that you've got the edge because of your legacy status.
Here's some data to support your view point. Note, however, that some schools only use legacy as a tie breaker. If the admissions officers would normally deny you, they might wait list you instead. If the admissions officer is down to two identical applications and only one spot, the legacy might win out.
So, list your legacy. Let everyone know. It's a not-so-secret advantage.
Legacy Admits: The Cons
You desperately want to go to CalTech or MIT. You've heard so much talk about how legacy students have an advantage in the admissions process.
There are many schools that do not honor legacies at all. They find the process of meritocracy unfair. After all, it rewards those students from a long-history of affluence over those who have just arrived to the game. MIT, for instance, does not honor legacies.
This video might help you understand.