isolated, closed and open systems
Three everyday containers tell the whole story. A perfect thermos lets nothing in or out — no heat, no matter; that is an isolated system. A sealed but ordinary jar lets warmth pass through its walls while keeping its contents inside; that is a closed system. An open mug lets both heat escape and steam drift away; that is an open system. The difference is simply what the boundary will let cross.
Formally, the three are classified by what can pass the boundary. An isolated system exchanges neither energy nor matter with its surroundings. A closed system can exchange energy — as heat or work — but not matter; the amount of substance inside is fixed. An open system can exchange both energy and matter. Truly isolated systems are an idealisation, since perfect insulation does not exist, but they are an invaluable thinking tool.
Choosing the right category sets up every thermodynamic calculation. The second law's statement that entropy never decreases applies cleanly to an isolated system; a closed system is the natural setting for the gas laws and most bench chemistry; and an open system is what you need for living things, rivers, and the atmosphere, where matter is constantly flowing through.
Your own body is an open system: it takes in food and oxygen, gives off carbon dioxide and waste, and constantly sheds heat. Seal it perfectly and life would stop within minutes.
Isolated, closed, open — set apart only by what their boundary lets through.
The labels describe the boundary, not the contents. The very same flask can be treated as closed when stoppered and open when its lid is removed; classify by what is actually allowed to cross.