For people working for pre-IPO companies that IPO-ed, how did the transition effect your shares?

As someone who has never worked in a pre-IPO company, how are shares awarded pre-IPO and how do they get translated post-IPO? Are they just internal shares or are they options? Is there some kind of internal share price that translates to a public one on IPO or is the IPO price decided at IPO based on evaluation?