Plain Old CLR Object
Plain Old CLR Object or POCO is a play on the term POJO, from the Java EE programming world (which was coined by Martin Fowler in 2000 ), and is used by developers targeting the Common Language Runtime of the .NET Framework. Simply put, a POCO object does not have any dependency on an external framework. It is PLAIN.
Similar to the Java context, the term is used to identify an object as a simple object, as opposed to the complicated, specialized objects that frameworks like ORM systems usually generate. Another way to put it is that POCOs are objects unencumbered with inheritance or attributes needed for specific frameworks  and are persistence ignorant objects. In .NET terms, the word is most often used in the programmatic sense, to differentiate a non-Serviced Component (see MTS) from a "standard object". It can also be used in a tongue-in-cheek manner, referencing the perceived complexity and invasiveness of Java-based programming frameworks such as the legacy EJB2.
Some benefits of POCO objects are:
- Allows a simple storage mechanism for data, simplifies serialization/passing around through layers.
- Goes hand in hand with depedency injection, and the repository pattern.
- Minimized complexity and dependencies on other layers. (higher layer's only care about the POCO's, POCO's don't care about anything). Loose coupling.
- Simple testability (since there's intentionally no/or little code in the POCO class, there's typically nothing much to test).
- See anecdote here: http://www.martinfowler.com/bliki/POJO.html
- See, for example, this MSDN article: Data Contracts - POCO Support
- See, for example, a reference to PONO in this whitepaper: Spring.net Reference Documentation