What are the important things to ask/know about them?
Thanks
You need to know how they affect your taxes. Go look up the actual regulations (they're not complicated), but it's roughly something like this: when they vest, that counts as income in the year that they vest, and your broker will typically immediately sell and withhold the money for taxes. When you sell them, you then have to pay taxes on the capital gains (the difference in the price you sell them for and the price when they vested). If you hold them for less than a year, you pay short term capital gains (basically the same as income). If you hold them for at least a year, you pay long term capital gains (15% last time I checked). Unless your company's stock price is rising incredibly fast, the vast majority of what is taxable is going to be on the vest, not the capital gains, so it won't make much difference whether you hold for a year or not.
Whether you should hold them or sell them is a topic of much contention and depends on the company and your personal situation.