The Yahoo distribution is a version of Hadoop 20 that they run (ran?) on some subset of their clusters. It includes a set of patches for stability, bug fixes, etc. It is a source release; it does not have admin-friendly features like rpm or debian packages, etc.
The Cloudera distribution is packages as rpms and debs (the source is also available). This means you can get updates via standard methods, etc. It also includes stability and bug fix patches. It is constantly maintained (not to say Yahoo's isn't -- I suppose one could just go on github and check when they last updated it). It also packages Pig and Hive.
Cloudera's distribution of Hadoop 20 is in beta, and 18 is considered stable (more on this on the Cloudera blog). The 18 version also includes packages for Hive and Pig; for 20, you have to build them yourself (there aren't official releases of Pig or Hive that support 20 yet, although patches exist). There may well be significant overlap between the Cloudera and Yahoo versions of 20; both provide manifests, so you can check. The latest documentation of Cloudera's distros is at http://archive.cloudera.com
Yahoo does not provide support for their distribution; they provide their patched version as a service to the community, so the folks who are interested can build what Yahoo runs internally. Given the size of Yahoo clusters, that's a significant contribution, especially if you aren't a Hadoop developer who follows the JIRAs all the time. Cloudera supports their distribution commercially, as well as providing some community support via the Hadoop mailing lists and, for distro-specific issues, on their GetSatisfaction page.
Both are pretty different from the vanilla Apache distro since they patch it in between releases (the cloudera version of 20 has 60+ patches!).