Simply: a belt for kimono; but an obi is so much more than that. The obi is a decorative, but necessary accessory for kimono wear. Japanese taste is to choose a colors in an obi that don't necessarily match the kimono, but compliment it by using opposite colors or focusing on a non-dominant color. There are several types of obi distinguished by material, weaving technique, shape and length. Obiage and obijime are also part of the kimono ensemble--these also work for a robe belt in a pinch!
In the movie Memoirs of a Geisha meiko (geisha apprentices) wear obi tied with the long ends loose (more provocative), while geisha and other women wear them tied in smaller bows (more sophisticated). There are a few hundred ways to tie the bows in back--we can really only do one or two! Some of the books on our Resources page address tieing the obi.
Obi also make dramatic wall hangings, table runners and curtain swags. A few books on the Links page have some suggestions. Email us a picture of how you've used an obi for decorating and it could appear on this site!