Slack is a “freemium” team communication & collaboration platform.
It consolidates all your team communication and files across multiple web services all in one place, instantly searchable, available wherever you go. Slack is free to use for as long as you want and with an unlimited number of people – wow.

The way Slack works is that you create Slack “channels” for discussions, projects, groups and topics that your team collaborates on. Channels contain messages, files & comments, inline images & video, rich link summaries and integration with popular web services including, Twitter, Dropbox & Google Drive.