The shares are yours, but the company reserves the right to buy them back at the lower of the current price and price you paid for them, which it does when you leave.
http://stockoptioncounsel.com/blog/standards-ownership-canth...
http://stockoptioncounsel.com/blog/standards-ownership-canth...
I never saw anything like this when I worked at an early stage startup, and it's obviously something an employee who reads their employment agreement would ask to be taken out. I'd never work at a company with that clause.
On the other hand, this can lead to disaffected employees who are only staying with the job because they are waiting for their options to mature.