The Plot: Near a quiet, peaceful country village, OurHero? comes into possession of an unused farm.
That's it. What follows is basically open-ended, somewhere between SimFarm?, a CRPG and a DatingSim, as you use your finely-honed resource management skills to run and administrate a successful farm, participate in village life, marry, grow old and (presumably) die (at my current rate of 24 hours of play time per in-game year, I haven't got that far yet...).
There are many versions of the game, some offering more variation on the theme than others. Rather than simply order them by date, I shall divide them into "with" and "without" swanky 3D graphics.
The original, but (much like the early FinalFantasies) not the best. Most of the basic ideas are there, and this is certainly still a true classic, but the interface and predominantly brown colour scheme are somewhat offputting to those who have played the later ones.
No less than four versions for the old GameBoy; Harvest Moon works very naturally as a portable game. Haven't seen any of them, so can't say much more about them.
The latest version of Harvest Moon in English, and certainly the best so far by a long way. The interface is much more natural to use, the colours are bright and vibrant (although not on an old unlit GBA screen, unfortunately), and the game is far deeper than most of its predecessors (it is in fact almost identical to the PSX game below, but with simpler graphics). In spite of a slightly off-putting translation job (for instance, the old man at the sheep festival who seemed convinced he was judging cows... of course, this could have been deliberate), this game is probably the best introduction to the series one could ask for.
3D
Harvest Moon 64 (N64)
The first of the series to use the aforementioned swanky 3D graphics, this one apparantly suffers from having a fairly shallow game under them. More on it if I ever find a copy....
One of the stalwarts of the series, later remade for the GBA; it shares the merits described above, and adds swanky 3D gra... yes, well, you get the idea. Many consider it the best Harvest Moon game to date.
Harvest Moon: For Girl (PSX) (Japanese only, and likely to stay that way)
A remake of Back to Nature, in which the player character is female, and hence gets to date the male inhabitants of the town instead of the female ones (though having met them, I can't think why anyone would want to...).
A departure from the usual formula, this version actually features a plot of sorts, involving a village threatened with destruction by expanding corporate consumerism (or somesuch), and a correspondingly limited time period. And, of course, swanky 3D graphics second only to...
Harvest Moon: A Wonderful Life (GameCube) (Japanese only, for now)
The very latest game in the series, for the GameCube. Luscious graphics, incredibly ambitious, character models that age as time passes, children... the series may well be attempting to edge its way onto TheSims?' market share, albeit with cute console-rpg characters. No doubt both of the people in this country who bought a GameCube will have great fun with this, when it comes out...