Functionally, interior window is the transition and gap covering of the space between a window frame (jamb) and the interior wall covering (typically drywall). Aesthetically it is an outlining adornment like the frame and matting for a picture.
The trim material is available in various wood species, sizes, and profile detail. It can be finished to match the window finish or contrasting. These choices, like any other accessory will greater change the interior look of the window, wall, and entire room.
Here is a step by step guide for Houzz.com on how to replace the trim on your existing windows: