On the left, you will find 'Categories' and 'Channels.' Channels are usually grouped into Categories by topic or type, depending on how the server is created. In the image, this server's channels are grouped into topic categories. On the bottom are music and video channels where members of the server can use the voice/video chat features to chat, listen to music, or watch videos together.
On the right are other members of the same server who are online and have their settings set to show their activity status. We'll get back to activity status later on in the guide.