Maltego comes with several standard Entity types. In addition, more Entities can be installed through the Hub. When designing Transforms, any Entity type can be used (including those from the hub). For non-standard Entity types, you will have to include them in the Paired Configuration of the TDS.
To look up existing Entities in Maltego click on Entities in the ribbon and select Manage Entities. The Entity Manager window will open listing all existing Entities. To view Entity properties, click the three dots to the right of an individual Entity to open the Edit Entity window. Here you will find basic Entity information, additional properties and Entity settings.