Lots of people have tried to capture this distinction, I'm sure I'll do a worse job here briefly than you can find around, but for me the tell is how people spend their time, and an attitude.
Hackers in the sense that I mean it have an innate need to understand things deeply, and a tendency to value achieving this directly (e.g. do something, don't just read up about it). As a result most hackers with any real talent will have achieved an unusually high level of expertise/mastery in at least one, often a few, technical areas. This is a result of having really spent a lot of time with it, in ways that may look "obsessive" to others.
This is by no means restricted to software. Another common characteristic is a tendency to take things apart (physically or virtually) to see how they tick.
