Chromebook basically is a personal computer which contain Chrome Operating System(OS). This device was built for using when connected to the internet however, some of the apps stored in the cloud were capable of running in offline mode also. Chromebook is an example of thin client. I would explain you about the thin client. Thin client is something which is dependent on web application to provide the required functionality to the user. Chrome OS was based on the Linux-kernal OS to perform best in the web applications.
Chromebook were first shipped by Acre Inc. and Samsung from June 15th, 2011 and it was announced at Google I/O Conference in May, 2011 itself. However, Lenovo, Hewlett Packard(HP) and Google entered the market in 2013.
However, Laptops are diverse category of devices. Sometime it is called Notebook, Ultrabook or Netbooks. In those era when Laptops and Desktop was manufactured some of the laptops were having the screen size as the standard "A4" size. Those were then called Notebook. A Laptop and Notebook lately were designed and released with different OS inbuilt.