Sending messages in Slack is identical to SnoChat and is self-explanatory: type a message in the message box and press ⏎ (Enter) or the Send Button.
If you want to send multiple lines of text, press ⇧ ⏎ (shift + enter) to move to the next line.
There are some built in commands within a chat message that you can see by typing the / character in the message and a list of the available commands, with instructions, will be displayed.
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!
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 
.
Emoji Reactions
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.