In Appacitive, a Type describes the nature of an Object as well as it's structure and contents. Each Schema is the model representation of a distinct type of Object.
The structure is expressed as a collection of Properties. Constraints on content are enforced through rules associated with these Properties. However, it is not mandatory to add Properties to a Type. Additional data regarding a Type can be stored in it's Attributes and Tags.
For example, to represent a location Object, you may want to create a Location Type with Properties Name, Address, GeoCode and Phone Number. You can then define constraints such as a phone number cannot start with 555 or that the value in GeoCode must be of the Geography data type.
Since Appacitive uses a graph paradigm, a Type can be considered to represent the kind of nodes that can be there in a system. From a UML perspective, a Type can be considered to be like a Class. In a RDBMS system, a Type can be considered to be like the Headings of a table.