A landmark in PC gaming. Not to be missed.
Baldur's Gate is based on AD&D, or Advanced Dungeons and Dragons. Before you run away in fear of becoming a geek, know this system works quite well for making a fantasy game of this sort. It lets you clearly understand your capabilities, your weaknesses, and most importantly just what's happening in combat... that is, if you bother learning it. It's entirely possible to go through the entire game without understanding the underlying mechanics, but to me at least, that would be frustrating. Baldur's Gate does a good job of telling you what's going on and remains quite faithful to the AD&D system of things.
The game itself has you starting out with relatively humble origins. You are allowed to select your class from a number of choices, customize your spells and weapon proficiencies, and modify your appearance or gender. After this, however, (as it always seems to be) something tragic happens and you are forced to flee from safety into the dangerous regions of the Sword Coast. What follows is a truly epic storyline accentuated by a string of quests, not all of them necessary. You can essentially go anywhere within the expansive game at any time, but this isn't necessarily recommended, as some areas are notably more dangerous than others. As you kill things and get experience points, you character increases in strength, which is important as monsters get stronger as the game goes on. The path through the storyline is generally pretty clear, but it's pretty neat to be sidetracked on other quests. These are a good idea to do, since often they have rewards of equipment, money, or experience (in many cases, a combination of these).
Speaking of equipment, there's plenty of it. There are many weapon types, and also many armor types. There's a wide variety of magical equipment to be found, as well, and the game includes a sizeable number of spells to cast for each of it's spellcasting classes. As you travel, you can also have people join your party. It's very, very hard to beat the game with just one person (if not impossible), so it's a good idea to get some members you like as soon as possible. There are a lot of people that can join you, from a wide variety of alignments, personalities, and classes. As you travel, fight, and complete quests, they'll gain experience just as you do and you have complete control over their level up. The only downside to having them is that sometimes their pathfinding can be frustrating; they'll catch on objects, or each other, and get struck trying to get from A to B. All it takes is a bit of supervision to fix this problem, though.
Visually, Baldur's Gate is quite impressive. Fully-rendered towns look quite nice, and spell effects are quite good. Animations look appropriate and the myriad of sprites the game has all look great. There are many different monsters you can encounter, and equipment changes your character's appearance accordingly. The art design is very well done and suits the fantasy theme well. Aurally, the game is great too. There are a massive number of voice clips recorded for almost everything; they all sound very believable. The writing is excellent, too, and some of the things people will say are hilarious. Sound effects are good, and the music is superb. It features a sweeping, epic soundtrack that suits the theme of the game perfectly.
All in all, Baldur's Gate is the very definition of an epic adventure. It's entirely possible to play through it multiple times, or just stay on one character and try to explore everything the game has to offer (which is far more than just the main quest). One of, if not the, best PC RPGs released to date.