Corp was established to accept business contracts and pay taxes under business tax laws, rather than non-profit tax laws (the famous anecdote of IRS at one point in the early days tried to convince Mozilla don't worry about paying as non-profit). But as Mozilla continued to grow, they hired more people and formed a whole corporation where people are paid to work on Mozilla's core projects like FireFox and keeping Mozilla secured. The confusion often occurs at figuring out the outreach programs. Foundation does some outreach, but Mozilla Corps as a whole also do outreach program. For example, I think people who manage ambassadors are Mozilla Corp employees, but correct me if I am wrong. Furthermore, there are developers hired specifically to work on Foundation applications last. My info could be wrong by now...