Microsoft Teams provides Channels to talk to everyone, Chats for private conversations, and Meetings for video conferencing. You can keep track of these communications by viewing the Conversations tab.

Communicate with Your Team in a Channel

Messages added to a Channel’s Conversation are visible to all team members but can only be edited or deleted by the original poster. View the Conversations tab to keep track of important messages, @mentions, and replies.

Post and Read Messages (Video, 4m 14s)
Edit and Delete Messages (Video, 2m 30s)
Notifications and Settings in Microsoft Teams (External resource)

Communicate Privately via Chat

Chat messages are private and only visible to the Team members you choose.

Use Chats for Private Messages (Video, 5m 6s)
Make Audio or Video Calls from the Chat Section (Video, 1m 55s)
Chat in Teams (External resource)