Pretty much every developer in the world knows about Stack Overflow, searching for a
solution to a development problem in Google will almost certainly
throw up a pretty damn good answer on Stack Overflow.
Along will many other Q&A sites that are now available on Stack Exchange there is a
Careers site too.
Careers 2.0 is a
place to show off your awesomeness. And I don't mean your
awesomeness on Stack Overflow - I mean everywhere!
As Careers 2.0 is tied to Stack Overflow there are certain
aspects of it which are designed to promote your activity on Stack
Overflow.
Such as the tags you are active in:

Your Stack Exchange accounts:

Along with any "top answers" you feel that are exceptionally
good you need to show them off.
And that is it - the rest of your profile is up to
you.
I will go through and show you some of the main sections that
are on your Careers 2.0 Profile to demonstrate that most of this is
nothing to do with Stack Overflow.
A personal statement
Add whatever you like here - it's entirely up to you. Just make
it good!
Add the technologies you are interested in and if you want, the
technologies you aren't interested in.
A number of the sections that relate to technology allow you to
choose the same tags that Stack Overflow uses when you post a
question. I am guessing this makes it easier for Employers to
search through candidates who like a particular technology.
Experience & Education
Fairly self explantory to be honest. You add your previous roles
along with a short description, time frames and the technology you
used.

This is where it can get interesting. You are able to to add
links to any open source projects that you have worked on. This
allows you to put your real projects out there on display and make
it easy for potential Employers to find and immediately see how you
would normally write code. Remember, this is code that you have
written properly, not some pseudo crap that you have quickly
knocked together so you can earn a quick 20 or 30 rep on Stack
Overflow.
Again more opportunities for you to show off your real
acheivements.
Writing & Reading
The Writing section allows you to link to
Amazon for any books that you may have written and also to any
articles or blog posts you may have written that you feel will make
you stand out from everybody else.
The Reading section is pretty similar and lets
you link to Amazon for any books you have read and again any blog
posts that you have read.
Obviously, you should pick and choose the relevant books that
you have read. I'm pretty sure potential Employers do not need to
know that you have read the "Shoot Annual" from 1983 :)
There are other sections that you can complete on your profile
along with some other nice pieces of functionality but I don't want
to spoil all of the surprises!
I get the impression that a lot of people think that Careers 2.0
is very closely tied to Stack Overflow and more specifically to the
reputation that you have on Stack Overflow. This is simply not the
case and I hope you can now see why your reputation on Stack
Overflow is a pretty small factor when it comes to your Careers 2.0
profile.
It seems I am in good company as Joel Spolsky blogged
about why reputation on Stack Overflow isn't what Careers 2.0 is
about.
Oh, and here is a link to my
Careers profile should you wish to have a look :)