Feel free to substitute "tag line" for "slogan and "brand" for "positioning" as Mark Ramsey, the president of hear2.0 clears it up nicely in this excerpt from "Positioning is not a slogan".
"What does it mean when a marketer asks you how you position your radio station to the audience? Twice today, stations have assumed I meant to ask "What's your slogan?" when that isn't what I meant at all. I don't really care what your slogan is.
"What I want to know is what, exactly, your station is supposed to stand for? What is it designed to represent? What do you want your listeners to believe about you and why do you think they choose you specifically over scores of other options as their favorite? What is the problem you uniquely solve for your audience?
" . . . A 'position' is not a set of words, it's a destination in the minds of the audience. "