England were unable to pull off a miracle comeback to surpass Australia's comeback in the Second Test, and they lose the Third Test and thus the series -- the Ashes return to Australia.
Explanation for most people reading this: The Ashes is a series of five-day matches played between England (actually England & Wales) and Australia. They play five 5-day matches. To take the Ashes trophy away from the current holder, you must win the series outright. The result of a cricket match can be a win for either side, but can also end in a draw (not necessarily a tie, which is quite rare; a draw is when the time limit on the game runs out before all of the players have been put out.*). So, for instance, in the five matches, you might have three draws -- if the other two matches are split, then the team currently holding the trophy retains it. England held the Ashes coming to Australia. This match in Perth was the third of five, and Australia had won the first two. By winning this third match, they clinch the series and take the trophy away from England.
There will be two more matches ("playing for pride"), but the overall result of this year's Ashes series is now settled.
I find I enjoyed watching cricket, now that I understand how the game is played. But I won't be sorry to get to bed at a decent hour tonight, instead of trying to stay up until 2 AM watching the final session of the day.
*You want to know more, go look it up, and I apologize to those of you who do know how the game is played for mangling the explanation in an attempt to simplify it.
Update, 23:30: Fixed airline reference.