You can send messages on the chat bar, also known as the message box.
Button[]
You can attach images, videos, and any other files, create a thread, use apps, and send a voice message (only on mobile). Do this by clicking the 
button on the left side of the chat bar. Additionally, you can also drag and drop files directly into Discord. You may add a caption immediately after upload. All files uploaded cannot exceed 25 MB, unless you have Discord Nitro, where it is increased to 50 MB with Nitro Basic or 500 MB with Nitro.
Message Box[]
Type words to chat with other users or enter a bot command. Press enter or the send button to send.
Markdown[]
Markdown is used to format messages using special characters.
Gift[]
You can send a Nitro gift using real money. It expires after 48 hours (2 days) if someone does not claim it.
GIF[]
Send gifs using the GIF Picker.
Sticker[]
The sticker picker lets you access over 14 default sticker sets as well as custom stickers.
Emoji[]
The emoji picker lets you access a large variety of default emojis as well as custom emojis.