The program Photoshop, by Adobe, is widely considered one of the most powerful tools to edit images. The problem for most home users is that it is $650 for the basic edition. This cost rules out most home users buying this program.
Most people, when they get a digital camera, will get some sort of program to do image editing, but they are often very limited. In addition, Windows does come with a program called Paint. Paint is very limited as well.
However, for those of you who are interested in stepping up to a higher level of photo editing, there is a free program called GIMP (GNU Image Manipulation program). I've personally used this program since 1999, and it is right up there with the expensive programs in terms of flexibility.