We all know that the backlinks increases the pagerank of a website. Do outbound links affect PageRank?
Outbound links do not increase PageRank. They do "pass" PageRank to the pages they link to, but they don't "take it away" from the page the link is on. You can learn more about how PageRank works at Wikipedia.
As far as page ranking goes, Matt Cutts said linking to external sites can be a positive ranking factor. (But this only affects a page's ranking, not a page's PageRank.
Outbound links do NOT increase PR, and this is obvious, otherwise it would be easy for each of us to increase our's website PR to infinite, wouldn't it?! We all simply add tons of outbound links to our sites. :)
But more important: it's true when John Conde says "Outbound links do NOT take away PR from the page they are in", but be very careful because in practice outbound links will end up reducing the all PR of your site.
I'll explain: being A,B,C pages on your sites, let's say page A has a PR of 4.
If page A links only to B and C it will give them about a 2 points of PR each [note 1]. Therefor you have a site where page A has a 4PR, B a 2PR and C a 2PR.
In the moment you add couple of outbound link to page A toward pages of an external site (i.e. a link to Wikipedia and a link to Facebook), page A does not lose its 4PR, but the PR passed out in each link will be only 1 point now!!!
Therefore now you have a site where page A has a 4PR, but B and C have both a 1PR, and NOT 2 anymore. This means page A will rank the same as before, but page B and C might now rank worst than pages (with similar contents) on you competitors' sites, in simple words your all site now might rank worst than before.
[note 1] this is just an example, we are supposing the PR passed out by a page is made by the page's PR divided by the number of links on the page, but the actual Google algorithm is more complex than this and according to Matt Cutt's a weight is applied to each links when passing PR, see the 2nd figure, that shows how otherwise a simple page loop would generate an infinite PR.