What is Ubuntu's *-series?

I read that u-series Ubuntu release that will be delivered in October 2014, designated 14.10 is not derived from another series. What is this u-series and what are the other series? Is there going to be a major change in the kernel architecture or something else?

Solution 1:

At this time there is no release codename for Ubuntu 14.10 yet but following tradition after the Trusty Tahr (which was the "T"-series) there will be a codename with letters "U", hence the interim name "u-series".

Also see: How are release codenames chosen?

It would be rather speculative to say which changes will actually land in the 14.10. release but it appears that Mir will be replacing X-Server, we will see Unity 8, and Covergence there.