Solid, basic chatroom
Slack is effectively IRC plus videoconferencing. It's an effective chat tool that lets you wrangle small groups to discuss specific topics or share files across plaforms more efficiently than texting, but is fairly no-frills beyond that. Channels are relatively involved to set up and only provide a single conversation thread, so attempts to incorporate larger groups (50-100 people) or use it for more wide-ranging discussion descend into unreadable chaos quickly; and beware the user who's discovered they can use @channel to make their posts stand out in the deluge. It's a solid option for coordinating among small groups, but there are many equally workable ones out there integrated into project management and social media platforms that may offer more for specific needs.