Roster Questions

I have always watched Hockey but mostly just Playoff Games. I really am wondering how the Roster works with contracts. I knew about the 23 man active NHL Roster and then I have read that there is a 50 Man and then a 90 Man. Could someone please explain what the differences between the 50 and 90?? Also how many players would you think the Knights would still need to Trade away to get to having 23 on NHL Roster with younger unproven players filling out the remaining 50 and 90 spots. As I am guessing we still have more than 23 NHL Caliber Players which would want to be on NHL Roster and not in a minor league system.

I am a 1st time Hockey Season Ticket Holder and look forward to meeting some of you.
I will be in Section 209 Row P Seat 16.