Question 893224
An expression is like a phrase or a clause; a function is like a sentence.

One can evaluate an expression; and one can evaluate a function for a given value for an input variable.



Another attempt:


An expression is a number.  The number can be written with plain numbers and variables, or it can be written just in variables.
{{{x^2}}},  {{{4(1/2)-3x}}},  {{{x^3+13}}}, or any such way of writing a number is an expression.


A function is, for your purpose, a statement usually using symbols and numbers, that some number with a name is equal to an expression.  The name for the function is given a letter like f, or g, or h, or sometimes other letters.
f(x) or g(x) or h(x) is a name of a function.  A function is often written as an equation, for example:
{{{f(x)=(x-3)^2}}} or {{{f(x)=(x+10)/(2x-1)}}} or {{{g(x)=x^2}}}.
The left side of the equation is a number.  The right side of the equation is a number.


We can use functions to say that a quantity (the name of the function) depends on the value of a variable.
If we have a function like {{{f(x)=x^2}}}, then this means the value of f depends on the value of x.  
-
If we want x=-3, then we can find {{{f(-3)=(-3)^2}}}, which means {{{f(-3)=9}}}.
If we want x=7, then we can find {{{f(7)=(7)^2}}}, which is {{{f(7)=49}}}.