Habitat
A habitat is the area where an organism lives, including the abiotic and biotic factors. In an ocean, this could be a fish and everything around it: the water, the other fish, the plants.
Niche
A niche includes the conditions where an organism lives and how they use them. So in a desert, this could be the relationship between the cacti, the amount of rainfall, the weather, and the other organisms around them.
Image sources:
http://animalsandhabitats.files.wordpress.com/2011/07/habitats.gif
http://images.nationalgeographic.com
/wpf/media-live/photos/000/001/cache/cardon-boojum-desert_102_600x450.jpg
http://animalsandhabitats.files.wordpress.com/2011/07/habitats.gif
http://images.nationalgeographic.com
/wpf/media-live/photos/000/001/cache/cardon-boojum-desert_102_600x450.jpg