i always say I dislike my mom and dad but I love them. So maybe liking someone is liking certain things or aspects about them, while love is unconditional. You love someone because you can't help it. Because you've been close for so long, and have had so many experiences with them, that you cannot do anything but love them.
You're probably thinking loving a relative is different than loving someone who isn't related, but I think its much the same. Like Csuti's friend said willingness to sacrifice alot of things for that person is part of love. Its really hard to pin point.
to clarify, you like someone for their positive characteristics, but you love someone for simply existing no matter who they are. Awful or kind. ugly or beautiful.