Slack is a team communication tool. That means it has slightly more robust features than a simple chat app, like threading conversations or channels for specific topics.

Slack is not open source.

Slack is not end to end encrypted.

Admins of a Slack could hypothetically read private messages between members of the Slack.

Free slack has a limited on how many messages it stores in it's history and it can have trouble scaling as the number of people using it grows.

But slack is also free, relatively generous and very familiar to many people so it has certain social advantages.