My marriage counselor said that (healthy) attachment is knowing someone will always be there for you if/when you need them, even if you don't see them/talk to them very often. (He was saying it in the context of me possibly wanting a future individual session with him.) I figure it's also like, say, a good friend who you don't see very often but who you know would be there for you if you needed them (and vice versa).

I imagine that unhealthy attachment and dependency are very similar. More of a neediness, being unable to handle things without the other person. Certainly some level of dependency is normal (like how a child is dependent on their parents), but if it's too intense, then it could be bad.