The YouTube inbox is a horrible piece of software. You won't realize how bad it is until you've used it for a little while. I'm here to try to help you use it with the least pain and suffering possible (there will always be some...). The inbox is always sorted by time the message/notification/request was received. It is is split up as such: Inbox: Inbox combines all other categories. A quick tip is to bookmark it for easy, regular access: http://www.youtube.com/inbox?feature=mhee&folder=messages#inbox/1 Personal Messages:The personal messages should be checked regularly - every week at least. It helpfully collects things like fan mail and network offers into one place, and unhelpfully includes a health dose of spam. On older channel layouts you can send personal messages from their profile by clicking the down arrow then clicking "send a message" On the new One Channel layout you can find the message link on the "About" tab (Thanks APunkGaming!) Comments: Comment on Videos:One feature of the inbox which is both very useful yet most confusing is the way YouTube collects video comments. A notification is sent to the channel when a video has unread comments, including the content of the most recent comment. This means that if one video gets 10 comments between when you check the inbox, only one notification will appear. This is not a big issue if you visit the video page to respond to the most recent comment which was displayed in the comment notification, but if the comment displayed in the inbox didn't deem a response, you likely won't visit the video page, potentially missing out on other comments and ways to engage with your fans. There is no way of knowing how many new replies a video got directly from the inbox. Replies to comments: These are replies to comments you made on other videos, they are included under the comments section. Contact notifications:Contact notifications are people requesting to add you to their 'address book'. This generally only allows them to spam you. I wouldn't recommend accepting them unless 1) you know who they are and 2) they are a channel you specifically are interested in working with in the future and 3) you plan on sending them or receiving messages from them in the future. Video Responses:Video responses are requests for users to allow their videos to appear under your own video as a response. Channel settings can be changed to "automatically allow" "allow with permission only" or "never allow". Generally you shouldn't allow people to post these to your videos as they will typically be fairly spammy. I keep mine set to "Allow with permission only" and very rarely approve requests. Sent:Sent messages. Zzz... Address Book:This is where those contacts are stored. Also where blocked users are recorded and such.