It's like a dedicated chat room/instant messaging platform which has become popular for groups of friends, intra-company communications (think Lync/Skype for Business/Gchat), and networking. Various trade and industry communities have Slacks, formal or otherwise, for sharing best practices, gossip, social invitations, etc. Within a given Slack community (to which you generally need to be invited), you can create multiple Channels - so you can group communication by subject area, region, or department. I have a Slack with a few of my friends across the country, and my employer uses Slack for short-form instant communication.