mantra, slogan, tagline

IBM: "Think"
Apple: "Think Different"
HP: "HP. Invent.
Google: "Don't be evil"

http://www.crews.org/curriculum/ex/compsci/8thgrade/company_slogans.htm

http://en.wikipedia.org/wiki/Motto
A motto is a phrase meant to formally describe the general motivation or intention of a social group or organization.

http://en.wikipedia.org/wiki/Tagline
A tagline is a variant of a branding slogan typically used in marketing materials and advertising. The idea behind the concept is to create a memorable phrase that will sum up the tone and premise of a brand or product (like a film), or to reinforce the audience's memory of a product.