Well, you can check out the System Resource Document for the d20 system, which is pretty much fully compatible with Dungeons and Dragons 3rd edition. You can find links to the SRD at
http://www.opengamingfoundation.org/srd/ . It's not the same as say having the Player's Handbook layed out in front of you, but it might help you get a feel for the system. I do recommend getting the new D&D Player's Handbook, it has brought a new vigor to D&D that it has been lacking since the early 90's.
To go to real basics, the system hasn't changed that much. You still have races, classes and so forth. Clerics still cast divine magic, Wizards still cast arcane spells. They added a few new character classes, namely the Barbarian and the Sorcerer, and renamed the Thief class to the Rogue class. The Barbarian is a mixture of 1st edition Barbarian and 2nd edition Complete Guide to Barbarians. The Sorcerer is all new, though most folks I know have been playing with something similar for quite some time.
Most of the changes have been to simplify and quantify the system in a consistent manner. All resolutions, from Thief skills, to attack rolls, to profiencies, use the same basic system, a d20 roll plus a modifier, the higher the modified roll the better. Saves were condensed into three rolls, and they now all make sense.
You have a Fortitude roll for attacks made on your physical being, such as poison or energy draining, a Reflex save to avoid things, and a Will save to shrug off mental effects. The saves, like other resolutions, is based on a d20 roll. Your individual Save, such as your Fortitude save, is a modifier to that d20 roll. So you might have the following saves: Fortitude +2, Reflex +1, Will +3.
A thorough and workable skill system has been added, with everyone getting them. Each class gets a certain number of skill points to spend per level, and each point buys a +1 modifier to your skill check. There are more complications to the skill system, but that's the simplified version. Like other resolutions, skills too use a d20 roll + a modifier.
There are no more demi-human level limits. Humans now simply hold an advantage in multi-classing versus other races. Yes, that's right, Humans are now the masters of multi-classing. No more dual-classing nonsense. Basically, if you want to have another class you take your next level in that class. So you might be a human fighter level 2. This means you are a level 2 character with 2 class levels in fighter. When leveling up to level 3 you decide to grip a level of cleric, making you a level 3 character with 2 class levels in fighter and 1 class level in cleric. You receive the full bonuses for both classes, hp's, attack bonus, skills, saves, excetera. You are a Fighter2/Cleric1 character and proud of it. One note is that Never Winter Nights will only allow you to take 3 different classes, to a maximum of 20 levels.
Your favorite six stats are still there, and they mean about the same things. Strength is how strong you are, Intelligence is how smart in book learning and cleverness you are and so. They have simplified the bonuses though. All stats now use the same table:
Stat Modifier
2-3 -4
4-5 -3
6-7 -2
8-9 -1
10-11 -
12-13 +1
14-15 +2
16-17 +3
18-19 +4
20-21 you get the point
This makes everyones life easier for remembering what does what.
While the standard D&D game now uses the 4d6 take highest three arrange stats to tast method, Never Winter Nights will use the system that the RPGA uses for their Living City Campaigns, namely the point buy system from the Dungeon Master's Guide. You get 28 points to buy your stats based on the following prices:
Stat Cost
8 -
9 1
10 2
11 3
12 4
13 5
14 6
15 8
16 10
17 13
18 16
The consequence of this is that you will be using alot of characters with the following stat numbers: 14, 14, 14, 14, 10, 10. This is one of the most efficient use of your points in this system. If you don't believe me, run the numbers yourself. 14 is the highest stat you can buy without paying a premium.
I hope this all helps.