Here is a list of all documented namespaces with brief descriptions:

DATA	The namespace DATA has been removed from LCIO all data entities are now defined in EVENT
EVENT	The namespace EVENT holds all base interfaces of the event entities
IMPL	The namespace IMPL holds the implementations of the base interfaces that are defined in EVENT
IO	The namespace IO holds the base interfaces for io of data
IOIMPL	The IOIMPL namespace holds extensions to the default implementations needed for IO
lcio	The LCIO namespace combines EVENT, IO, IMPL and UTIL for user convenience
lcrtrel	The namespace lcrtrel holds classes and functions for runtime extensions and relations see LCRTRelations for more detailed documentation
SIO	The SIO namespace holds the persistency implementation using SIO
UTIL	The namespace UTIL holds convenient and support classes and methods for the LCIO objects