A political slogan generally expresses a goal or aim ("Workers of the world, unite"), whereas an advertising slogan is most often intended as a memorable identifier ("The real thing").
Slogans vary from the written and the visual, to the chanted and the vulgar. Often their simple, rhetorical nature leaves little room for detail, and as such serve perhaps more as a social expression of unified purpose, rather than a projection for an intended audience.
Slogans are attractive particularly in the modern age of informational bombardment from numerous media sources (see also soundbite, meme).
Slogans are a core part of propaganda (see also Newspeak).
"Slogan" comes from sluagh-ghairm (pronounced slogorm), Scottish Gaelic for "battle-cry" , as in the popular movie Braveheart.