Cash is a limited resource a team has per season. Each team gets to spend $100 each season (which they can add to or subtract from via trades, but they do not get any more in any other ways).
Cap Space is the amount of money a team has under the Salary Cap. Cap Space is calculated as such:
Cap Space = Salary Cap – Team Salary – Cap Hits – Dead Money
So, if a team has 20 players on its Active Roster worth $82, and they have a $1.5 Cap Hit and are holding $2.5 in Dead Money, their “Cap Space” would be: $100 Salary Cap – $82 Team Salary – $1.5 Cap Hit – $2.5 Dead Money = $14.