After you perform certain actions in your IMAP client for the first time, you'll notice a few new labels when you next log in to Gmail's web interface. For example, sending a message in your client creates a 'Sent Messages' label in Gmail, indicating which messages you sent through your client and not through Gmail's web interface.
Some actions that will result in new Gmail labels are: creating drafts, sending and deleting messages, moving messages to a junk mail folder, and creating new folders in your client. Note that the specific names of these new labels will vary depending on which client you're using.
If you create folders within folders in your client, Gmail will treat these as separate labels. Gmail indicates which folder is a nested folder by adding a '/' before the corresponding label in the web interface.
Please note that some folder names are 'reserved' as special system folders in the Gmail web interface. If you attempt to create a folder name that conflicts with a reserved folder name such as 'Chats' or 'Sent,' these folders will display with an '[IMAP]' prefix ('[Imap]/Chats').