Channels are personal space for every registered YouTube user and are required for some features of the site. A YouTube channel is designed to show a short description of the channel, display thumbnails of videos you've uploaded, videos from other members you've picked as favorites, subscribers and a section where other people can comment on your channel if you choose to allow that.
Up until 2010, YouTube was using the Channels 1.0 or the original channels. They were really simple and to the point. The left side of the channel contained user's info and information about connecting with that user, and possible subscriptions and subscribers. On the right side of the channel, there was a featured video and below that, a list of other uploaded videos and then the comments section. There was a set of links at the top of the channel. It was easily customizable.
YouTube introduced a new channel layout in 2010 simply known as Channels 2.0. The layout was slighty different however it still kept all the customization options that the previous version had. It offered more branding for partners, however it got a negative feeback by the community.
Channels 3.5, codename Cosmic Panda
On March 7, 2012, YouTube migrated every channel to the new channel design known as Cosmic Panda. It got the version number 3.5, because the version numbers of the four start page designs and three channel designs were synced. However, it is now mostly referred as Channels 3.0. The new change got a long of negative feedback by the community as the only customization that could be done is to change the background on a regular user's channel, however partners could add a banner at the top, but no colour scheme can be changed whatsoever. 80% - 99% of the people voted against the new channel layout and oftenly hated it, however, this long hate stopped when one channel was announced.
Channels 4.0, codename One Channel
On February 4, 2013, a new design was announced. It instantaneously received criticism because of the lack of customization, the too similar layout to the main page, and the change in general. In March 8, 2013, it became available for users, however, users were still able to use the Cosmic Panda layout until around June, when it became manatory, this resulted in users quiting their accounts, note this is the current design of the channel layout, there is no longer a way to revert back to the cosmic panda layout.