No offense, but google would have given you the majority of those answers. But since I'm nice:
Netbook: Small, portable computer that is capable of basic tasks like web surfing, word editing and HD content viewing (If configured correctly).
Advantages: Small, light, efficient and cheap. Most have rated battery lives of over 6 hours and have weights around 3 or 4 pounds.
Disadvantages: Small screens and keyboards as well as typically lower performance. Netbooks tend to come with screens that range between 10 and 12 inches. These are tough to read for some people. Netbooks tend to come with underpowered CPUs (Atoms and Neos) as well as weak integrated graphics (unless they are based on NVidia's ION platform).
Price: Generally anywhere from $250-$600 (USD) depending on manufacturer and model.