The classes in this namespace are .NET wrappers for ObjectARX AcDB* classes. The general naming rule is to replace the AcDb prefix on the ObjectARX class with the namespace. For example, AcDbObjectId becomes Autodesk.AutoCAD.DatabaseServices.ObjectId. The following overview is extracted from the ObjectARX Developers Guide (<Map 3D SDK root folder>\docs\arxdev.chm). The topic path is ObjectARX Introductory Concepts/Database Primer/AutoCAD Database Overview.
An AutoCAD drawing is a collection of objects stored in a database. Some of the basic database objects are entities, symbol tables, and dictionaries. Entities are a special kind of database object that have a graphical representation within an AutoCAD drawing. Lines, circles, arcs, text, solids, regions, splines, and ellipses are examples of entities. A user can see an entity on the screen and can manipulate it.
In the preceding paragraph an entity is a DatabaseServices.Entity object and a database is a DatabaseServices.Database object.
Symbol tables and dictionaries are containers used to store database objects. Both container objects map a symbol name (a text string) to a database object. An AutoCAD database includes a fixed set of symbol tables, each of which contains instances of a particular class of symbol table record. You cannot add a new symbol table to the database. Examples of symbol tables are the layer table (AcDbLayerTable), which contains layer table records, and the block table (AcDbBlockTable), which contains block table records. All AutoCAD entities are owned by block table records.