The release notes contain the significant changes in each PostgreSQL release, with major features and migration issues listed at the top. The release notes do not contain changes that affect only a few users or changes that are internal and therefore not user-visible. For example, the optimizer is improved in almost every release, but the improvements are usually observed by users as simply faster queries.
A complete list of changes for each release can be obtained by viewing the CVS logs for each release. The pgsql-committers email list records all source code changes as well. There is also a web interface that shows changes to specific files.
The name appearing next to each item represents the major developer for that item. Of course all changes involve community discussion and patch review, so each item is truly a community effort.