what exactly is RSU? What's the difference between it, and stock you buy from a broker on the market?

An RSU is like an option with a strike price of $0. You receive the option to puchase stock (with vesting).

This is in contrast to actually purchasing a stock on the market.

> to purchase stock [via] vesting

So when the rsu has vested, it is converted to normal stock, where you can sell on the market for cash?

> converted to normal stock, where you can sell on the market for cash?

With the smaller Google-specific "well but" limit that insider trading rules restrict employees from trading in the company stock except for narrow trading windows after the company's quarterly reports.

You can sell during blackout periods if you file a 10b5-1 plan a few months (year?) in advance.

http://www.investopedia.com/terms/r/rule-10b5-1.asp

Yes. It's just stock on a vesting schedule.

Yes, google GSUs = RSUs.

Yes, thanks. Brainfart.