$\bullet$ To determine if an expression is a function, we perform the vertical line test.

$\bullet$ Surjective/Onto: For every $y$ value, there exists at least one $x$ value.

$\bullet$ Injective/Into/one-to-one: For every $y$ value, there exists at most one $x$ value.

$\bullet$ To determine if a function is one-to-one, we perform the horizontal line test.