DCO stands for Dyalog Component Objects. It's a concept implemented in the mid-nineties useful to encapsulate functionality. It also provided a standard of how to document modules. These modules used namespaces and could be saved on file.
Over the years it grew to a large library of modules, covering many areas.
With the introduction of OO in version 11, however, DCOs became an outdated concept. Existing DCOs are maintained until 31.12.2010, but without any enhancements.
On the long run all DCOs are supposed to be replaced by classes; for details see: