The client-side Glide API is partly a subset of the server-side API, but it also provides APIs for dealing with forms and lists. These APIs allow for the control and manipulation of the behavior of elements of the ServiceNow interface, and interaction with the database, user, and session information.
Many client-side APIs don't provide constructor methods, or require instantiation. For example, rather than declaring a new instance of the GlideUser class, we simply have access to the g_user object.
One of the most important things to understand about client-side code is that it can have serious impact on the user experience, in terms of performance. Querying a record from the database, for example, requires constructing a query, sending it across to the server, waiting for the response, and then waiting for all of the data associated with the response to that query (and all of the records contained therein) to be sent back to the client from the server.
While all of this happens...