A good way to remember it is the acronym "DEBT" Something with Protection cannot be:
Damaged by whatever it has protection from (color, spells, converted mana cost of 3 or less)
Enchanted by whatever it has protection from (color, etc)
Blocked by whatever it has protection from
Targeted by whatever it has protection from.
A lot of people
forget the enchanted part. If a creature has protection from a color, then it can't receive enchants of that color. This also means that if you give it protection from a color, all enchants on the creature of that color fall off.