The human eye can only retain images for roughly one-tenth of a second. Because of this, when we see images played quickly, one after the other, our brains blend them into a single moving image. In other words, an animation!
However, there are some factors, such as Frame Rates, that make animation more believable.
Frame Rate
Each image displayed is known as a Frame, and the number of images shown every second is known as the Frame Rate. A low frame rate (for example, 12 per second) might make an animation look choppy and unrealistic. Higher frame rates make animation look smoother, to the point where you might actually believe that the pictures are real.
To give you an idea of how much work is involved in commercial animation, Disney's classic animated films run at a frame rate of 24. That means that there are 24 images (frames) shown every second. The original Jungle Book is about 80 minutes long, meaning there were around 115,000 images created for that film!