Any crud left on the crocks at the end of the washing cycle will be broken down food debris. Rinse Aid is vitally important and so is salt. It helps from time to time to clean the filters and the jets in the spray arms and run a cleaning tablet through the machine now and again.
Following two paragraphs from Wikipedia...
The heat inside the dishwasher dries the contents after the final hot rinse. Rinse aid is used in the final rinse cycle to allow water to run off items and prevent water droplets forming.
If the dishwasher has run out of the salt that recharges the ion exchange resin that softens the water, and the water supply is "hard", limescale deposits can appear on all items, but are especially visible on glassware.
Peccavi, June 2009
