Like the ones who you can really depend upon. The ones who will love you, nurture you and care for you unconditionally. Who would be there if you had a car accident and were hospitalised, would take you in if you became homeless, would come running if you went through a bad break-up.
Been doing some thinking recently and it's shocked me to realise that there are only 3 people (excluding family members) who i'd count as proper friends, as described above.