The Baldur's Gate games and the Icewind Dale games both use the Infinity Engine (just slightly different versions depend on which game you play in each series) so they are very similar in terms of how you play the game. I'll list some key features to give you an idea of what they are like:
- Both are mainly single player and are played without an internet connection, although there is a multiplayer option which allows you to play over the internet or a network.
- They are based on AD&D rules just like NWN
- You control your party using the mouse - in fact the only keyboard keys that I actually use are the SPACE to pause the game and TAB to highlight doors and containers etc.
- You can pause the games anytime, either by clicking on the pause icon using the mouse or pressing SPACE, including in combat, and so you can take your time deciding which spell to use and so on. You can give the commands to your party then just unpause and let them carry them out.
- They are set in the same world as NWN
- They are "2.5D" rather than 3D, in the sense that you look down from an isometric viewpoint and the camera is fixed unlike with NWN
- The storylines are far more absorbing and well written than those of NWN and the quests are far more diverse
- You control an entire party, rather than just one character
- In the Baldur's Gate series you have a wide choice of NPCs that can join your party, each with their own skills and personality, as well as loves and hatreds for other NPCs and so on. They also banter with your character and each other throughout the games, have their own quests and so on, and are a lot more "real" and in depth than the henchman in NWN (and you have complete control over their actions once they join your party)
- In the Icewind Dale series you create an initial party of up to 6 which you then use for the entire adventure
Hope this answers some of your questions. If you have any more then feel free to ask. I would highly recommend these games as all four (BG1 + expansion, BG2 + expansion, IWD1 + expansion and IWD2) are some of my favourite games of all time

I also enjoy NWN and while it has somethings extra (like more feats and so on), it largely doesn't even get close to competing with the Baldur's Gate games as far as gameplay, story and replayability.
Lastly, if you use a program called BGTutu (you will find lots of info about this on these forums) and own both of the BG games (1&2, with or without expansions) then you can play BG1 using the newer version of the infinity engine that is used by BG2. (While BG1 is definately a lot of fun without Tutu, Tutu makes it more fun for me, although other players find that they prefer BG1 exactly as it was intended to be played. Its just a matter of preference really).