WebP is a new image format like JPG, PNG etc. developed by Google. Currently one of the main reason for slowness for web page loading is due to fact there are lot of images on the page and the the size of image is also large too.

This format would allow image files on the Web to be smaller in size yet not loose any of its 
sharpness. It does so by using  better compression algorithm than currently used by PNG and JPG of the world. Also it can display the image in a progressive mode without waiting for the entire image to be
compressed and decompressed. 

These images are natively supported in Google Chrome.  Google expects that more main stream browers like FireFox,Safari, IE etc. would provide support for this format once this gains traction.