This new PostgreSQL database feature adds over ten thousand lines of new code to the open-source server and allows the write-ahead log stream to be decoded into a series of logical changes, per this commit.
PostgreSQL's logical decoding feature is more elegantly described via this mailing list post:
This feature, building on previous commits, allows the write-ahead log stream to be decoded into a series of logical changes; that is, inserts, updates, and deletes and the transactions which contain them. It is capable of handling decoding even across changes to the schema of the effected tables. The output format is controlled by a so-called "output plugin"; an example is included. To make use of this in a real replication system, the output plugin will need to be modified to produce output in the format appropriate to that system, and to perform filtering.Look for logical decoding to appear in the PostgreSQL 9.4 release.
Currently, information can be extracted from the logical decoding system only via SQL; future commits will add the ability to stream changes via walsender.