Data is simply a word used to describe any 'information', usually stored in a digital format. A 'leak' is when that information is made available to users that shouldn't have access to it, usually through some sort of security breach or flaw.
'Sensitive' information or data is usually sent via secure email channels, ones that are encrypted and difficult to intercept.
However, anything from human error through to espionage could result in an email data leak which could mean the spreading of that information to unauthorized recipients.
Just look at the high profile 'Wikileaks' or Edward Snowden cases to see what kind of impact email data leaks can have on the world around us.
