Really, it depends on context. A sign is something that gives a message. ie: a stop sign. A mark is something that holds a sound or a meaning as a base or is something that has meaning that 1: changes on perspective or 2: is only known to certain people. A symbol is a mark that is given a important meaning or some sort of importance. Sometimes religious symbols. ie: a crucifix, Japanese Hiragana, Chinese Kanji, ect. As for +, *, @, these are symbols because they hold a specific meaning.