Meat and Milk: Meat and milk (or derivatives) may not be mixed in the sense that meat and dairy products are not served at the same meal, served or cooked in the same utensils, or stored together. This leads to the requirement to have separate sets of dishes and cooking utensils for meat and milk and a requirement to wait after eating meat before consuming dairy products. There is a variation in custom of how long one must wait, anywhere between one and six hours.
Meat and Fish: While meat and dairy products and their derivatives may not be mixed or eaten together in any amount, 'pareve' (neutral) products can be mixed with either meat or dairy products. Fish is an exception: it may not be mixed with meat.
Milk and Fish: Some are accustomed not to have milk and fish together on the same basis that meat and fish may not be consumed together.