To put it a little more explicitly. Ubuntu names their releases for the YY.MM of release 16.04 was released in the year 2016 (16) in April (04). So those are

To put it a little more explicitly. Ubuntu names their releases for the YY.MM of release 16.04 was released in the year 2016 (16) in April (04). So those are date based releases rather than semantic versioning (semantic is x.y.z with x major, y minor, z patch). They release a LTS version every two years in April: 12.04, 14.04, 16.04 and the next will be in 2018, 18.04. They do add a "patch" version after that, but the YY.MM are the major releases. 16.04 is an older version than 16.10, but it will be supported with patches for a longer period of time (major versions of individual software packages will stay the same).