Notebook - 1.5 hour battery life, heavy, about 700-3000 dollars, more features, etc...
Netbook - 3 hour or 6 hour batteries, light, under 300 dollars, no CD/DVD drive, 2-4 USB ports and SD memory card slot.
A netbook is a great compliment for a PC, but a notebook is a good (though pricey) replacement.
The largest issue netbooks and notebooks have are keyboards I find; watch out for any irregularities.
I use a PC/netbook/smartphone (Acer PC, Acer Aspire netbook, and Apple iPhone 3G) combo which I find is excellent; cost me about $1000 total for all three (cheaper than my TV) and has an upkeep of $100 a month for Internet on my computers, and phone/data plan - and that price typically falls each year (my Nokia N95 used to cost me $130 per month alone just two years ago).