When a challenge is made all of the words created by the previous play are challenged. The challenger doesn't have to specify which word or words ze are challenging.
When a player wants to challenge, they say hold before their opponent has selected their tiles, the clocks are stopped and the player has to decide whether or not to challenge. After 15 seconds their opponent may select their tiles, but they must keep them separate.
From Wikipedia: https://en.wikipedia.org/wiki/Challenge_(Scrabble)
Bingo!
What does a Bingo-prone rack look like?
Most or all of the tiles will be one or two points in value. Larger value tiles can be part of a Bingo, but it is more restrictive. There will be a balance of consonants and vowels. If the rack is heavier with consonants, then look at the board to find a vowel to use. There will be few if any double or triple tiles of one letter.
Play
The first question to ask is, "Is this a bingo prone rack?" If it is, then make a play that scores well and keeps letters that work well together and will hopefully form a bingo next turn. Making a rack more bingo prone almost always means playing your high-value tiles. This will keep a good leave to cultivate a possible bingo.
You can still score well even if you can't play a bingo. If you are able to play your power tiles on the bonus squares - especially if played two ways - you can score 40, 50, 60 or more points.
Kenji Matsumoto does a very thorough analysis of when it might be correct to pass [http://www.breakingthegame.net/passing] I have seen other endgame and pre-endgame variations of his examples, but by and large this is a great write-up.