From it you can start a conversation with any of the users if they are connected. In case they're not, you can also leave them messages so that they can read them later and answer you at any moment.
From the small interface of Hangouts you can also choose if you want to save the conversation history with any friend in particular, if you want to disable the notifications of a user or if you directly want to send the whole conversation to the archive. You can also block a user.
Hangouts is an instant messaging tool for your browser that's quite useful and complete and, despite of being a bit minimalist, offers all the benefits you need to communicate with your friends or colleagues from work. Additionally, you can follow the started conversations in the browser, using the Hangouts version for Android or iOS (also available in Uptodown).