They are two ways of stating the same number. RTP is the percentage of wagers a game returns to players over the long run; the house edge is what is left over. A 96% RTP is a 4% house edge.
The arithmetic
House edge = 100% minus RTP. A slot at 96.2% RTP has a 3.8% house edge. A European roulette wheel at 97.3% RTP has a 2.7% edge. Blackjack played with correct basic strategy runs around 99.5% RTP, so roughly a 0.5% edge.
There is no hidden difference between the two terms. Game makers prefer RTP because 96% sounds generous; mathematicians prefer house edge because 4% is what actually happens to your money.
Why the framing matters
"96% RTP" invites you to think you keep 96% of what you stake. What you actually lose is 4% of everything you wager, not 4% of what you deposit, and the difference is large. Deposit £100 and play through it twenty times, which happens quickly at slot speed, and you have wagered £2,000. The expected loss is £80, not £4.
That is the mechanism behind the common experience of depositing a modest amount and watching it disappear at a game with a "high" RTP.
Where the edge is smallest
Blackjack with basic strategy, then video poker on a good pay table, then baccarat banker bets, then European roulette. Slots sit further out, and the house edge on side bets and on American roulette is materially worse than the main game.
What neither number tells you
Volatility. Two games with identical RTP can feel completely different, because one pays small and often and the other starves you then pays big. That is covered in our RTP and volatility guide.