I picked this up from Guy Kawasaki's blog, the full list can be found on Make Marketing History.
1) Marketing is not a department.
Marketing is a combination of elements that creates the environment in which it is possible to meet a customer need (starting right back at product development). Promotion and sales are just sub-sets of marketing.
2) Marketing is a conversation, but most people don't speak geek.
Successful technology marketing must translate the creations of the uncommunicative into the needs of the untechnical. Spin is not good marketing. Lucid two-way communication is.