From the 4e D&D PHB, p 179:
In contrast to an obstacle that requires one successful skill check, a skill challenge is a complex situation in which you must make several successful checks, often using a variety of skills, before you can claim success in the encounter. . . .
Whatever the details of a skill challenge, the basic structure of a skill challenge is straightforward. Your goal is to accumulate a specific number of victories (usually in the form of successful skill checks) before you get too many defeats (failed checks).
From the 4e D&D DMG, pp 72, 74 76:
An audience with the duke, a mysterious set of sigils in a hidden chamber, finding your way through the Forest of Neverlight - all of these present challenges that test both the characters and the people who play them. The difference between a combat challenge and a skill challenge isn’t the presence or absence of physical risk, nor the presence or absence of attack rolls and damage rolls and power use. The difference is in how the encounter treats PC actions. . . .
Define the goal of the challenge and what obstacles the characters face to accomplish that goal. . . .
Level and complexity determine how hard the challenge is for your characters to overcome. The skill challenge’s level determines the DC of the skill checks involved, while the grade of complexity determines how many successes the characters need to overcome the challenge, and how many failures end the challenge. . . .
What happens if the characters successfully complete the challenge? What happens if they fail?
When the skill challenge ends, reward the characters for their success (with challenge-specific rewards, as well as experience points) or assess penalties for their failure. . . .
Begin by describing the situation and defining the challenge. Running the challenge itself is not all that different from running a combat encounter . . . You describe the environment, listen to the players’ responses, let them make their skill checks, and narrate the results.
When the characters overcome a skill challenge, they earn the same rewards as when they slay monsters in combat - experience and perhaps treasure. The consequences of total defeat are often obvious: no XP and no treasure.
Success or failure in a skill challenge also influences the course of the adventure - the characters locate the temple and begin infiltrating it, or they get lost and must seek help. In either case, however, the adventure continues. With success, this is no problem, but don’t fall into the trap of making progress dependent on success in a skill challenge. Failure introduces complications rather than ending the adventure.
This is all pretty clear, as clear now as it was a decade ago: a skill challenge is the resolution of a particular challenge/situation that arises in the fiction; the players (and their PCs) succeed or fail based on their checks made within a "clock" framework of successes-before-failures; and the success or failure is just that: either the players (and PCs) achieve their goal within the situation, or they do not.