Declaration simply means initialization without value. for example: let x; Expression is the operation being used to some primitive data types and will return a value result, like: Numerical expression (10+30) String expression ("Hello" + "World!") Boolean expression (a > b) Conditional expression (age > 18 ? "allow":"exit") while Statement is just a fancy word to combine both, like: let x = "Hello" + "World!" // this whole line is called statement