I've gone to a few FNM drafts, and it's structured as three qualifying matches, followed by a single-elimination tournament for the top 8.
Do all these games affect your DCI rating?
Magic tournaments are in the process of changing from using DCI Rating to "Planeswalker Points", which strongly rewards players for simply playing Magic (with DCI rating, NOT playing was sometimes best), while still rewarding skill/winning, although to a much lesser extent. You can read all about the change in this article on the Magic the Gathering website.
In spite of the change, the answer still stands:
Yes, Playing in Friday Night Magic events earns you Planeswalker Points.
The Planeswalker Points website says that FNM is a 3x multiplier on the points you earn. Event multipliers range from 1 to 12:
Yes they do, but they have less impact on your rating, and a less serious level of rules enforcement, than tournaments at a higher level of play.
From the Wizards site:
Here's how that 8k figure factors into the calculation of your rating change after winning/losing a watch:
For comparison, there are lots of more competitive tournaments at the 16k level, and Premium tournaments are at 32k!