1. How do "skill challenges" work?
At its heart, boiling everything down into the essential bits, all a skill challenge is, really, is the group accumulating so many successes on a series of skill checks before accumulating three failures. The more successes you need, the more difficult the skill challenge.
There a whole lot of variation and a whole lot of wrinkles, such as using some skills to open up the use of other skills, some skills being easy to use for a given challenge while other skills being hard, skills giving bonuses to rolls, and so on. Succeeding at a skill challenge means the group did what it needed to do, while failing usually leads to more complications, such as a combat encounter, losing a healing surge, or whatever.
Skill challenges are designed to dovetail with the combat system, so you could create a skill that's the equivalent of, say, a solo monster and make it worth the appropriate amount of xp. They can be used for social or physical endeavors (interrogations, wilderness trekking, negotiations, etc.).
Page 42 in the DMG is a discussion on actions that the rules don't cover. There are some guidelines for how to treat unusual actions by the PCs and a table with various skill difficulties and damage expressions by level. If a PC tries a stunt and pulls it off, then the DM can use the suggestions on this page to help determine the level of damage the PC does. The example given is a PC swinging on a chandelier, kicking an ogre in the chest, and pushing into a brazier of burning coals.
The whole spirit of this page is that the DM should encourage PCs to perform stunts, think creatively, and how to do that from a game mechanics perspective. My favorite passage from this page is:
Your presence as the Dungeon Master is what makes D&D such a great game. You make it possible for the players to try anything they can imagine. That means it's your job to resolve unusual actions when the players try them.
That just neatly encapsulates what page 42 is about.