When dial up Internet was a thing, AOL was the leader in that universe. It was basically one of the earliest walled gardens out there. They had their own chat, email, etc. You accessed it all through their desktop client. You never had to leave the AOL application to experience "the internet". They of course were still providing general Internet access, so you could still fire up Netscape [and earlier browsers] and cruise to whatever websites you wanted. The Internet eventually eclipsed what AOL could offer for most people and their premium price and sometimes dodgy Internet service eventually fell by the wayside to a more open Internet experience for most users.

Edit: AIM was a really popular messaging client for years after the fall of AOL as a premiere dial up Internet service.