Retail value is the highest value, think of it as being when the car is retailed on a car dealer floor. This is the average value you will buy the car from any car dealer.
Trade value is the lowest value, think of it as being when you take your car into a car dealer and the sales person makes you an offer on your used car. This is the average value you will sell your car at any dealer.
Market value is simply the average of the two above, so the average between Retail and Trade value. This is the value if you were to sell your car privately considering the mileage, condition, service history, accident records and overall condition compared to the condition when the car was new.
There is also one other type, the agreed value which generally applies to unlisted, vintage and collectors' cars where a valuation is given by an appropriate approved source.
What do these mean when insuring your car?
Should your car be stolen without being recovered, or written off in an accident, the Insurers settlement amount i.e. the money they give you to buy another car, will be based on one of the above value types.
When insuring your car it is always good to know which value type the policy will be based on, since the higher the value to replace the car, the higher the monthly insurance premium is likely to be.