Release Cycle
Ubuntu has a release cycle that seems awkward at first, but makes a lot of sense on closer inspection. Every release has a YY.MM scheme, where YY is the two digit year and MM is the two digit month. Hence, Ubuntu 16.10 was released in October of 2016. Releases occur every year in April and October.
Now, there are two different types of releases: standard and LTS. Standard releases are supported until the next release. While these represent the latest features in open source software, users wishing to keep their machines secure need to upgrade every six months. The LTS releases, on the other hand, are supported for five years. LTS releases occur on April of every even numbered year.
So the currently supported LTS releases are 16.04, 14.04 and 12.04. The current standard release is 16.10.
Flavors
There are several different flavors of Ubuntu, each with its own default desktop environment:
Ubuntu Desktop: Unity 7
Ubuntu Gnome: Gnome 3
Ubuntu MATE: Mate Desktop (fork of Gnome 2)
Kubuntu: KDE
Xubuntu: XFCE
Lubuntu: LXDE
Now each of these flavors have something in common: They are all meant to be installed on desktop PCs. There are three other flavors of Ubuntu worth mentioning, however:
Ubuntu Server: Used on full scale servers with SAS RAID, 10 GbE, etc...
Ubuntu Core: Intended for embedded devices that have no display.
Ubuntu Touch: Intended for smartphones.
The Future
If everything goes according to plan, 18.04 should mark the release of Ubuntu Personal, which is the convergence of the Desktop and Touch code bases. The desktop will be Unity 8. While this may seem asinine at first, I can elaborate on why they are doing it. It is slated to be the biggest release they have ever made.
And while this last part hasn't been confirmed, it also seems as if Ubuntu Server and Ubuntu Core are going to be combined.