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In computer programming, an _expression_ is something that yields a value. 

A _statement_ performs an action.

For example, let us look at some pseudocode. Let's assume that we want to calculate the sum of 3 variables:

    sum = a + b + c;
    print(sum);

`print(sum);` is a statement: it performs an action.  
`a + b + c` is an expression: it yields a value.

Now you may be wondering: is `sum = a + b + c` a statement, or an expression?  
The answer is that it's a statement, but it _contains_ an expression. `a + b + c` yields a value, and then an action is taken: the value is assigned to a variable.  

In this example, we have an arithmetic expression. But most operations on strings and booleans are also expressions!  
For example, we could have a conditional:
     `if (a > 3 && p == 5) { ...  }`
In this condition statement, the part `a > 3 && p == 5` is an expression. A boolean expression.

Or, we might be concatenating strings:

   fullName = firstName + " " + lastName;

In this line of code, `firstName + " " + lastName` is a string expression. It yields a value. The line as a whole is a statement: it evaluates an expression and stores the result in a new variable.

In general, expressions occur inside statements. An expression yields a value, but after you have your value, you'll want to do something with it - store it somewhere, or output it, or send it as an argument to another function.