Slack is an instant messaging server. Groups of users can share a subdomain of slack.com and create unlimited rooms (channels) on each. Slack also provides proprietary clients, in particular the webchat and some mobile apps.
Designed for synchronous communication, Slack also has some features of asynchronous communication. It has won accolades for his apparent ability to supplant other communication systems both synchronous and asynchronous, largely because it allows users to be very noisy and spammy: "There's great power in the ability to ping anyone at any time. There has to be a lot of respect for a lot of people".
As a fully centralised service, Slack gives zero power to its users on how their chats are run. Unlike IRC or Matrix, there is no way to run your own server and federate it, no way to handle your own authentication or identity provider.
Slack also follows no open standard, but rather its own "Real Time Messaging API", for which some official and freely licensed library exists. It's not clear whether the level of documentation for the API allows it to be considered an open protocol, but it's certainly not a standard in that it's not adopted beyond Slack.
Depending on a single entity in USA for an entire chat system presents obvious technical and legal risks, for instance Slack took the liberty to terminate any user who was possibly Iranian, without any warning. This affected all sorts of persons and groups outside Iran who need unfettered global communication.
Slack is notoriously impervious to archival: even retrieving your own personal messages for preservation is essentially impossible. Slack fuels the illusion that chat history is always available, by making it easy for users to login or reconnect after any period of time and read all the messages they missed, and by providing some functionality to search chat history.
Any message sent to Slack must be considered to be lost forever after it's read, unlike for instance on IRC channels and servers which have plenty of log services. Slack isn't suitable for any message or content you or others may want to retrieve in a month or a year from now. It might be ok as a substitute for a phone call (you wouldn't rely on either side of the phone call to record or transcribe the call, even in contexts where it's a standard practice).
As of 2020, it's not clear whether the GDPR demands for data portability will make a dent in this lock-in system.