Laptops - A generic term for any portable computer that can be used on your lap etc.
Notebooks - Again a generic term interchangeable with laptop, they started to be called notebooks with the reduction in physical size from the early briefcase sized units like the IBM PS/2 Portables. There is a story that Dell started to call their portables Notebooks after somebody had their tackle burnt by the heat from the underside of one of their units, though this is likely an urban myth.
Ultrabooks - A reasonably recent addition, these are very compact, very thin, light weight highly portable machines, generally without optical drives and featuring SSD storage, the PC manufactures answer to the MacBook Air.
Netbooks - Small compact "mini" notebooks, normally featuring less capable hardware than their bigger cousins with screens up to around 10-11" lower power consumption means the battery life is greater than that of a fully featured notebook, generally used for surfing the net, social media and email etc. Hence the name.
Tablets - Thin hand-held devices (though I have an HP TC1100 from 2005 and believe me that isn't thin, nor easy to hold in one hand for too long) generally with touch screen interfaces, low power consumption etc.
At least someone doesn't (ignorantly) append "homework" tag to any thread out here.