A Kindle is an e-book reader produced by Amazon. Each time Amazon brings out a newer generation Kindle, they change the device and add new features. It is important to know what generation you have, in order to buy compatible accessories. For example, a Kindle 2 cover will not fit on a Kindle 1. Every generation allows book downloads directly from Amazon, but they differ in how the user can get online.
Look at the back of your Kindle. If you find an SD card slot, you have a Kindle 1.
Measure the display size. Most Kindles have a 6-inch screen. If you have a 9.7-inch screen, you have a DX.
Inspect the keyboard. If the toggle is built into the keyboard, you have a 3rd generation. If it is higher, close to the Next Page button, you have a 2nd generation.