Being in love is sort of like an obsessive disorder. You think about the other person constantly, or at least for thirty minutes out of every hour. Any reminder of the person -- a photo, a small gift, an affectionate e-mail, a message on your answering machine -- makes you smile and start thinking about the person all over again.
It feels good. It's not exactly a "high", it's more like a really good radio station that you listen to all day long for as long as you like. It's happiness with virtue. It's not permanent, but anyone who has lost it wishes it could be.
There may be other forms of love which you actually meant to ask about, however --
Erotic love is, well, lust. It's desire for a sexual high which the other person provides. If it's unsupported by true love, it only lasts as long as the novelty of the relationship and the sex does, which can be a single weekend to several years. It has a lot in common with a drug high, in that it's addictive, it's obsessive, and the crash you suffer after your "fix" is cut off hurts a lot.
True love is commitment. It's sacrificial. It's about making another person the highest priority in your life, so that not only would you do anything this person needed, you wouldn't even have to think about why you're doing it. (This is not the same as doing anything this person would want, mind you. They have another phrase for that kind of relationship.) You aren't in love with the person on a constant basis; that would be exhausting. But certain things remind you on an irregular basis that you are in love, which is better than being constantly in love because of the element of chance.
Marital love is a large dose of true love combined with a few spoonfuls of familiarity, a cup of ongoing lust and a lot of communication. You can approach marital love with an SO or a live-in boyfriend/girlfriend, but to really get the full effect you need to stand up in front of all your friends and family to say that you intend to spend the rest of your life with this person and mean it. It's more work than true love by itself, but the momentum builds up faster than you'd think. Most days you wake up thinking "I'm in love with this person" without attaching a lot of emotion to the idea, but every once in a while you two get to do something together that brings all the old feelings right back. This is worth the wait.