Suntan lotion is for tanning. It only blocks
enough UV light to prevent sun burns, but enough to get a more
tan complexion with enough sun exposure.
Sun screen is a more
casual protection. It blocks more UV light than suntan lotion,
but is a light enough formula that it does not alter the
appearance of skin while it is on.
Sun block is a formula
which blocks significantly more UV light; however it tends to go
on very thick. The smell of it also tends to linger even after
you rub it in. This formula is ideal for being in full sun, such
as when on the beach.
Check this LINK
for SPF information.