Image hosting is a service that allows you upload images and photos to the Internet. This is so you can share your images with your friends and family or you can just use it as a place to put images that you need to link to on the Internet, for example in auctions, blogs and forums etc . . .
The image host that you upload your images to will usually store the image on it's own servers and then provide you with various ways in which to link to the image for viewing. So basically, Image Hosting and Image Uploading are ways to get your images online without having to pay.
"Because many users do not have personal webspace, either as a paid service, or through an ISP offering, Image Hosting services have become massively popular as the demand for hosting services increases. With the explosion of the popularity of blogs, forums, and other interactive pages, image hosting services have become extremely popular among end users." - Wikipedia