So, on to the actual theories (if you're still with me):
If you take the biological view, love is like hunger or thirst and satisfies the same end goal as lust: child production. Lust makes you take notice of potential mates, romantic attraction makes you focus on a mate, and attachment leads you to actually tolerate your mate and the child long enough to rear it! Not exactly a glorious explanation of love though it may be an opinion that you, or one of your characters, might hold. Characters who take this rather pragmatic view are probably less likely to lose their head or even fall in love in the first place. They may well choose their mates with a bit more of a practical eye. Of course, people can be the biggest cynics in the world and get bitten by the love-bug but they're less likely to have many romantic 'illusions'.
One of the leading theories on the psychological side, is Sternberg's Triangular Theory of Love. Basically, love is made up of three different things: intimacy (the extent to which they confide in each other alongside feelings of bondedness), commitment (expectation of permanency and shared achievements and plans), and passion (the heart hammering and sexual side of things).
Nonlove is a sense of absence of all those positive qualities of love. The partners may fake an attachment in public but generally have little to do with each other.
Liking/friendship is a sense of truly bonding with someone, feeling close to them, and enjoying their company, but without any intense passion or even long-term commitment. While they may be quite upset to learn the other will be leaving soon, they can readily come to terms with it, because it's simply the loss of a companion that bothers them and not the loss of a future together.
Infatuated love is the pure, whirlwind passion that is most commonly found in novels, particularly Fantasy novels involving squabbling characters with nothing in common. This can generally disappear quite quickly as it lacks the long-term commitment or even the deep understanding and bonding with the other partner to sustain it. Of course, over time, infatuated love can become another form of love as other aspects develop. Some people can become quite addicted to the heady experiences of infatuation and jump from bed to bed in pursuit of it. Generally, this form of love requires more than a one night stand - if the passion is high enough to warrant the title, then it's probably high enough to sustain itself over several nights, at least, if not weeks or even months. This is often a highly sought after love to experience on holiday as it lends a pleasant quality to the entire experience.
Empty love has neither intimacy nor passion to give it any real sparkle. It may be that a stronger form of love has dwindled over time or the individuals have had their marriage arranged for them or that they married for individual or communal gain.
Romantic love doesn't involve any long-term commitment but does involve a lot of sharing of pasts, hopes and dreams, as well as a passionate enjoyment of each other's company. This is the Western ideal of the start of the relationship. In fact, if most romantic movies and other such tales are to be believed, this is the Western ideal full stop as it's rare that long-term commitment gets a look-in. Heart rates go up at the sight of them, hours are spent in eager conversation, and people are eager to meet and greet again.
Companionate love is similar to friendship love but far more lasting as there is a long-term commitment of shared goals and values. It's lack of sexual desire makes this form of love quite looked down upon in modern, Western society as a relationship whose flame has gone out and some view it as hell to be stuck within it. However, while the passion may have gone out (or never existed in the first place) there is a deep affection and commitment leads to a strong bond of its own. This form of love might be found also in the platonic close friendships and family relationships that people have.
Fatuous love is rarely explored and it probably should be. This is probably what happens after all those Fantasy novels including hateful characters bedding each other probably winds up. Commitments are made (normally by the end of the book so as not to bore us with the lack of will-they/won't-they) after a whirlwind courtship and marriage. Trouble is, they've never really gotten the chance to get to know each other. Perhaps they preferred not to or simply didn't think of it at the time.
Consummate love is the so-called complete form of love as it comprises all three of the aspects in relatively equal dimensions. Their passion for each other hasn't dimmed (they're the eighty-year-olds that make the rest of us squick), they know so much about each other, and they can't imagine being with anyone else. They try to deal with their issues as they have an eye on the long-term but they do take a moment to indulge.
Of course, relationships can ebb and flow. Nonlove can become consummate love as the boxes are ticked off. An arrangement marriage can glow brightly as Empty Love becomes imbued with passion and intimacy over time.