Message Threads - NEW
Threads let you respond directly to a message in a channel, keeping the replies organized neatly in a single, threaded conversation. Go ahead, ask questions and dig deeper, have side conversations, or go on a tangent without interrupting the flow of conversation. Use message threads to:
- Encourage open discussion without distracting others.
- Organize conversations and preserve meaningful context.
- Focus your time and attention on the task at hand.
The quick instructions:
- Hover over the message you'd like to reply to directly.
- Click the reply icon. Or, click on any message with replies to view the thread in the details pane.
More information on threads - https://get.slack.help/hc/en-us/articles/115000769927-Message-threads
Notifying other users
To get someone’s attention in a channel, type @ followed by their username, for example @john. (They’ll see a highlight on the channel and be notified via email or their mobile device if they’re not online)
You can also notify everyone who is online in a channel by typing @here. You can also notify everyone (online or offline) in a channel by typing @all, but avoid using @all too often because it notifies everyone. Although @all is useful for important announcements, it can be distracting if abused, so it is recommended to use @here.
Deleting Messages
You can delete a message by clicking the 'three dots' icon in your message and then clicking the Delete Message item in the menu popover. A confirmation will be displayed before the message is deleted.
Editing messages
To edit a message, click on the three dots' icon on the message when you hover over a specific message, and then click on the pencil icon. Alternatively, in the message input box, hit the Up Arrow.
Switching messages
If the cursor is at the beginning of a message, hitting the Up Arrow switches to editing the previous message that you wrote:
Similarly, if the cursor is at the end of a message, hitting the Down Arrow switches to editing the next message that you wrote.
Pinning Messages
Pin the most important messages of the channel for everybody to see. Pinning a message to a channel causes the message to be easily accessible via the channel details menu. This allows for important information to be pinned to a channel, so people can easily see important information.
Starring Messages
Star your favorite messages for yourself. Starring a message is similar to pinning a message, however starred messages are personal and only you can see the messages you have starred. Starred messages are per chat room and are not global, each room has its own list of messages you have starred in that channel.
Emoji! & Emoji Reactions
To insert emoji in a message, hit the emoji button to the right of the text input to view an emoji picker. You can also type your desired emoji by wrapping its name with :; for example, :heart: produces 
.
You can also react to a message with an emoji. Hover over a message, click on the 'three dots' icon, click on the smiling face icon and choose an emoji.
To remove an emoji reaction, click on the emoji below the message. To reiterate an existing emoji reaction that someone else posted, click on that emoji to add to that emoji’s counter.