Macs and PCs come with basic software for viewing images. A simple way to organize these images is to put them into folders on your hard drive. However, the ease with which digital cameras and capture images means that the average user quickly has more images then they can organize in any meaningful way.
Media apps like iPhoto and Picasa are designed to make it easier to organize large numbers of images. Sites like Flickr and Picasa web make it easier to upload your images online, create collections and share with others.