A type of DistributedDataStorage where data is scattered among numerous peer nodes, which may  (or may not) be geographically distributed, and the role of a centralized authority/repository (such as a RelationalDatabase engine) is minimized or eliminated.

Advantages of peer-to-peer (many of these advantages are theoretical; as production-quality peer-to-peer systems that can compete with a BigIron RDBMS like Oracle simply don't exist).

* Greater scalability.  The Internet wouldn't work well if everything had to go through a central server somewhere.  Servers invariably become bottlenecks.
** Can be implemented on inexpensive hardware.  A RDBMS requires a piece of BigIron to run effectively.
* For applications involving multiple autonomous entities, better models the underlying political reality--each node is autonomous.
* Reliability and fault-tolerance; if one node goes down the rest still function.  (Though many peer-to-peer implementations exist where if one node or link goes down, down goes everybody... [''What P2P application on earth goes down if someone turns off their computer?'']).  With server-based systems, the server and the surrounding infrastructure is a SinglePointOfFailure.
* Better models the world; inconsistency is a part of real life.


And, the advantages of the server-based system (including things like replicated databases, where a single logical database is scattered across multiple systems).

* Much easier to achieve GlobalConcensus--the canonical state of the system is kept by the server; everything else is just a (possibly out-of-date copy).
** Likewise, data consistency is not an issue--the OneTrueCopy lives on the server.  With peer-to-peer, it is easy for the distributed data to become inconsistent.
** Easier to maintain invariants on the data and provide transactional support (including the ability to roll back transactions which cause an invariant to not be satisfied). 
* For applications involving entities under the control of one organization (such as corporations), better models the underlying political authority--the governance of the authority can implement policies for things like security, backups and data recovery, etc. at the server.
* The ''location'' of data is not an issue (it lives on the server).  With peer-to-peer, before you can find out ''what'' the data is you may have to find ''where'' it is.

Note that the issues discussed here are largely orthogonal to the RelationalModel--a NetworkDatabase is just as capable as a RelationalDatabase at addressing the above.  (However, the most mature DBMS products are RDBMS's).