Bet builders let you combine multiple selections from a single match — for example a result, a goalscorer and a card count — into one bet with combined odds. They're hugely popular for football and now offered by most top UK sites.
Accumulators (accas) combine selections from different events. Each added leg multiplies the odds but also multiplies the risk: every leg must land for the bet to succeed. Many bookmakers offer acca boosts or insurance-style promotions on multi-leg bets.
Treat builders and accas as high-entertainment, high-variance bets. Keep stakes small relative to your budget and see any success as a bonus, not an expectation.