When adding to the mash, white rice turns into maltotriose, maltose, and other simpler sugars, depending on the mash temp - like the rest of the starches. The major difference is that it doesn't have diastatic properties of its own - so you need to be careful about the ratios. Up to 25% is commonly considered acceptable, but it depends on the style. It does have a different (more neutral) flavour to barley.
Personally I'm starting to lean on (cooked) white rice in the mash-tun over adding sugar to the boil, as you don't have the potentially adverse affects (acetyldehyde mostly) of adding pure sucrose/fructose/dextrose.
If you want a cleaner beer, use rice. If you want a drier beer, use simple sugars.