Sia's first-party graphical interface is called Sia-UI. It is aimed at providing Sia's core functionality (file storage) to users in a friendly and simple way. Many of the more powerful features of Sia (such as choosing your own redundancy) are not presented through the UI in an attempt to make a user's first experience as simple as possible.
A tutorial for getting started using Sia exists here.
The Sia-UI features a full plugin system so that first party and third party plugins can be easily added which extend the functionality for more advanced users. Currently there are no plugins that have been created for the UI.