Ask HN: How do shares work?

I am relatively young but I have been involved in startups for the last year or so. None at the low level and none I have shares for. But that's about to change as I am co-founding a startup and we just got into TechStars for the summer and I feel like now is the time to learn about it.

I've been really curious about this for awhile and sort of know some things but I'd like a well rounding knowledge of it all so can someone tell or point me to some resources to be knowledgeable about all the funding and shares terminology and explanation. How do you go from having it split three ways between founders to Facebook having "millions" of shares? Is that completely different because of the IPO? What do all the fundraising terms mean (i.e. convertible, how valuations work, dilution, etc)?

I've seen people do math of acquisitions and them calculating what the founders would take based on dilution, etc, how does this work?