Glad you asked. =D
Harvest Moon is a farming game. Sounds boring, huh? Trust me, it's not. The basic plot for most games is that you are a guy (or girl) who's grandpa (mom in one case) died and YOU inherited their farm. So you go out there to plant seeds, raise animals, and marry one of the many girls (or guys). (The color of a person's heart determines how much they like you. When their heart gets to a certain color you can marry them.)
Of course some games are different in their plots.
In More Friends of Mineral Town you are bored with your life so you buy a farm, only to discover you've been ripped off. In Rune Factory you battle cave monsters to save the town of Kardia. In Harvest Moon DS you rescue the Harvest Goddess. In Island of Happiness, you are shipwrecked and half to turn an island into a community. In A Wonderful Life, your character actually gets older until, at the end of a period of time, they die. But in other games your little farmer is immortal. =D
There are always tons of characters in Harvest Moon, and they're always...interesting in their own ways. And every game has hidden features that are not mentioned in the instructions they give you, such as cursed items found underground.
Or, in simpler words, Harvest Moon is my obsession.