Tennis scores count from 0 (love) to 15 to 30 to 40. Score starts at Love-Love. This is NOT called Deuce.
"15 all" means 15-15. We can have 15-30, 15-40, 30-all.
40-40 is a special condition called Deuce. At this point, the next score is either to "Ad In" (server) or "Ad out" (receiver). If both sides tie again, the score goes back to Deuce. If the Ad In player gets another point, he wins. If the Ad out gets another point, then he wins.