Declarations: A declaration is a statement that creates something in your program. These are always at the top of your script file. They include variables, functions and classes.
Expressions: An expression is an operation you perform on data in your program to get new data back out of your program. Expressions can be mathematical operations like addition or division, or they can be something more complex like a function call (which takes input values as arguments, does some processing on them and then returns new output values).