System is the part of the physical universe which is under study, while the rest of the universe is surroundings.
Hot tea or milk (system) kept in a stoppered thermos flask remains hot for a couple of hours. If this flask is made of perfect insulating material, then there would be no exchange of matter or energy between the system and the surroundings. Such a system is called an isolated system.
Isolated system is a system which can exchange neither matter nor energy with the surroundings. Closed system is a system which can exchange energy but not matter with the surroundings. Open system is a system which can exchange both energy and matter with surroundings.