In order to keep kosher, one must keep dairy and meat products separate. In order to do this to the fullest extent, one must use separate cooking appliances for dairy and meat, as well as separate places to clean them to keep them from mixing.
According to Orthodox authorities, a dishwasher may not be used to clean both meat and dairy utensils. More liberal authorities claim that a dishwasher can be kosher and wash both types of utensils if it is made with a stainless steel interior, provided that the utensils are washed at separate times with a waiting time between and certain other procedures are followed. Some households avoid this problem by having two separate dishwashers.