As you already know, Chatter is a Salesforce collaboration tool that can be built into any record of your organization. This is automatically enabled on any organization created after June 10, 2010.

Posting to Chatter is easy with the Process Builder:

We can select to post to a User (by selecting a specific user or referencing an object field, such as OwnerId or CreatedById), to a chatter group, or to the current record Chatter feed.

The message can contain up to 10,000 characters with text, merge fields, already existing topics, and mentions (up to 25 per message).

To dynamically mention a user based on a record's field value, use @[{!fieldReference}] as the syntax; for example, @[{![Opportunity].OwnerId}].

Remember that if you use Microsoft Internet Explorer 11 (which will be officially deprecated at the end of 2020; see https://help.salesforce.com/articleView?id=getstart_browsers_sfx.htm&type=5 for more details), you won't be able to copy and paste text into a message, so be careful.

Finally, remember not to start the message with a field reference (for example, {![Opportunity].Name}) because that action won't be saved at all. This is a known issue – just add a blank space at the beginning of the message as a workaround.