Phil Sturgeon has a new post today that looks at the relationship between the PSR-0 standard (autoloading structure) and Composer - noting that they're friends, not relatives.
As a huge proponent of Composer, a happy user of PSR-0 and a voting member on the PHP-FIG I get into plenty of conversations about all of them and it worries me how much confusion there is in the community about these things not actually being related. [...] It seems that a lot of folks discover Composer and PSR-0 at the same time and seem to assume they are the same thing - especially since both Composer and PSR-0 have the idea of a "vendor" and a "package", but those two things are not related to each other either. These are a few points that I have wanted to clarify during some strange conversations over the last few weeks.
He goes on, trying to clear up some of the confusion around the idea of "vendors" and vendor names. He talks about naming schemes and how they may or may not be related to the vendor name on the package. He looks at the PSR-0 loading methods and how the structure of the library/repository effects that (noting that Composer can be made to accommodate something not PSR-0 by default). He suggests that PSR-0 needs to remain "implementation agnostic" and that Composer, at the same time, should remain "specification agnostic" .