Tabs are the title bar-like representation of a webpage - like title bars, they can be moved independently, but can also be grouped together to form a single window of tabs. Chromium's UI is based around the treatment of tabs and their content as individual top-level elements rather than as children of a parent window.
Ctrl+Tab, Ctrl+Shift+Tab, Ctrl+PgUp and Ctrl+PgDn can all be used to switch back and forth between tabs. While there is great demand for an MRU-ordered switcher, we've so far been unable to find an MRU switcher that makes sense beyond the first three most recent tabs, or one that works well with background-created tabs.
Rather than enforcing a minimum size, tabs continue to shrink as tabs are added - this was found to be more satisfying than forcing users to swap to a different tab management model at some arbitrary cutoff point. It is also assumed that users with a large number of tabs will be more able to manage those tabs, and future work (such as a tab switcher overlay) may help address those cases.
The tabs are depth ordered from left to right, with the active tab always in front - tabs do not maintain any other depth order (changing the depth sort based on focus order was more confusing than useful).