A fridge van is designed to carry goods that need to be maintained at a temperature in excess of 0 degrees centigrade: this would include things like fruit and vegetables, sandwiches and fresh meat, for example.
Freezer vans are built to achieve and maintain much lower temperatures of minus 18 degrees centigrade or lower. They are used for things like ice cream and frozen food.
Whether you need a fridge or freezer van depends entirely on what you are carrying and what temperature it needs to be stored at.