There are many classes of objects and requests that can be made to those objects. Objects are anything that you might work with in Avenue. Views, tables, and layouts are objects as well as strings or numbers that you can manipulate. Requests are specific to the object. Use the Arcview Help facility to find out which requests are appropriate for the object you are working with.
Classes are general pattern with properties and requests for a specific type of object.
A Grid Theme, a theme that represents raster data is an object of GTheme class. You can have any number of Grid Themes based on this class. Other classes include Views, of which your project might might have one or more view objects based on this class. The class defines which requests are appropriate to the object that you want to work with. There is an error checking utility called a compiler that detects syntax errors but not runtime errors.