The terms ABox and TBox are used to describe two different types of statements in ontologies. TBox statements describe a system in terms of controlled vocabularies, for example, a set of classes and properties. ABox are TBox-compliant statements about that vocabulary.
TBox statements tend to be more definitional in nature such as a dictionary of words.
TBox statements are sometimes associated with object-oriented classes and ABox statements associated with instances of those classes.
Together ABox and TBox statements make up a knowledge base.