When I started out as a business owner I used to keep it short and dull because I didn't want to bore people. "I own a marketing company" I would tell them.
That had two problems:
- it's incredibly boring and doesn't invite follow-up questions
- marketing is so broad that it's not informative
I've moved onto "I run a marketing business that helps small business owners to afford new cars and nice things". It's simple, has people imagining the real benefits of great marketing, gets a laugh and always gets a few follow-up questions.