Slack is a set of collaboration tools and services, where you can easily communicate with all your team members.
You can create different channels for different projects and topics, and then invite all the relevant people to join these channels. Then, you can send each other texts, images, videos and other important files.
Slack is especially useful for remote teams, because it enables communication as if all team members were in the same place.
For even better productivity, this tool allows you to integrate over 1,000 other tools, such as DropBox and Google Drive, within the basic Slack.