Embodiments of the invention provide methods and systems for defining classes of objects which entails defining and manipulating expressions. A two-tiered classification editor operates to enable a user to define and manipulate expression in real time through a browser-based user interface. The first tier comprises a group editor for enabling a user to define groups. The second tier comprises an expression editor for enabling a user to define and manipulate an expression comprising a set of groups and one or more relationships between the groups. Each group may comprise a set of conditions, each of which may be based on a possible attribute of an object. Via the expression editor, a user can drag and drop to create and modify these expressions on-the-fly. When a user is done manipulating an expression on the second tier, the user is returned to the first tier, which may present the expression as modified.