Rule #1. Most of the stuff called 'Neighbourhood's that you download are not they are terrains. Anything that has an SC4 suffix and/or goes into your SC4Terrains folder is a terrain. When you create a new Neighbourhood you select a terrain to build it on - two different things. Capisce?
Rule #2. Neighbourhoods go in your Neighborhood folder. Some people may have noticed the spelling difference there. No, I didn't make a mistake (either time). EA can't spell (we won't go into the whole Brit/Aus English vs American), it's a known fact - anybody who's had a look around with SimPE will be able to tell you that.
Rule #3. All new Neighbourhoods (by default) are created with the same NPC's and Townies as are in your C:\Program Files\EA GAMES\The Sims 2\TSData\Res\UserData\Neighborhoods\N001 folder (default Pleasant View template). By swapping out that folder with other (appropriately created) N001's you can get different 'default' Sims.
Further Reading List