Slack is a instant messaging application for teams. You can organize your team conversations either in open channels or Private Groups. To reach a colleague directly, you can send them a Direct Message, which is completely private and secure. You can simply drag and drop images, PDFs, documents, and spreadsheets into Slack and share with anyone. If you use any services like Google Drive, Dropbox, or Box, just paste the link and that document will immediately be in sync.