The English word 'love' is used to mean all kinds of things. The Ancient Greeks had four different words for concept which we refer to as 'love'. This makes defining love much easier, but doesn't complete the task since these four words have ambiguous meanings too. The Ancient Greeks debated themselves debated the nature of these concepts.
Agápe: Deep feelings of warmth and regard towards something; characterized by benevolence and general deep positive feelings rather than desire.
Éros: Passionate feelings of desire and longing, though not necessarily sexual.
Philía: Fellowship between friends, family and the community; generally revolving around virtues of loyalty, mutual respect and shared activities and interests, rather than passion.
Storgē: Natural affection and acceptance, usually used described relationships within families.