love:
there is no way you can adequately define love for another person as it varies from person to person. love is an attraction between to people through various interactions. i don't understand love, i still find the concept hard to grasp. but for love to develop you need to have an interest in the other people. the first action is to get to know the other people. this way you can understand them better, have a better judgement of what you feel about the other person, and whether these feelings are true. if after an encounter where both parties are aware of each others presence, affection may develop between them and hopefully a mutual desire for each other's company.
this affection can arise from a number of things, love at sight which tends towards lust. this desire is physical in the majority though, there are cases where there is more, such as chemistry. chemistry being, pheromones, sub-conscious pre-determined interest in a certain body structure and behaviour. this i believe would account for 'love at first sight'
the other being an affection developed through prolonged interaction, friendship or the like, where upon a situation or conflict may bring two parties closer together forming a deep affection for one another based on each others compatibility and mutual trust.
so i guess if you love someone you are deeply affectionate towards them, otherwise its just a crush....until you do something about it.