The Options screen is part of the user interface which allows you to customise various game options. The default key to access it is "o", and it appears on the left panel so cannot be viewed at the same time as any of the Character screen, the Achievements screen, or the or the Events and PvP Screen.
It has 6 tabs along the top, and buttons to save or cancel changes at the bottom.
Graphics
This section allows you to choose a graphics adapter, switch between fullscreen and windowed mode, and change resolution. It also has various graphical quality options.
Sound
This section lets you choose a playback device, and set volume levels for several different categories of sound.
UI
This section lets you choose default language, and turn on or off a variety of features such as map location/transparency, and the display of various optional features such as quest tracking and health bars. Options relating to the Chat console are also here. It is also the location of Item allocation options and Item filter options.
Input
This section lets you change keys for various commands, or restore your keys to default.
Notifications
This section lets you turn on or off various pop-up notifications.
Streaming
This section is used to set up Twitch streaming.