Ubuntu releases, like Ubuntu 17.02, 17.01, 16.12, etc, [ REF: https://launchpad.net/ubuntu/+milestones ] are monthly snapshots of progress made [ REF: http://askubuntu.com/a/800314 ] towards the next final release [ REF: https://wiki.ubuntu.com/Testing/ISO/Milestones ].
Ubuntu development uses a time based release schedule, rather than a feature based release schedule [ REF: https://wiki.ubuntu.com/TimeBasedReleases ]. Releases are numbered by the last 2 digits of the year, a period, and the digits of the month [ REF: http://askubuntu.com/a/1234 ]. In addition to monthly snapshots, there are also daily snapshots [ REF: https://wiki.ubuntu.com/Testing/ISO/Milestones ]. Final releases are considered finished products [ REF: https://wiki.ubuntu.com/FinalRelease ], but security updates, and bug-fix updates are provided to them [ REF: https://help.ubuntu.com/community/UbuntuBackports ]. In the case of long term support final releases additional point releases (eg: 16.04.1) are made containing these accumulated updates [ REF: https://wiki.ubuntu.com/PointReleaseProcess ]. This saves time when doing a fresh install, because updates will already be in place. Final releases are classified as either long term support releases (receives updates for 5 years), or normal releases (receives updates for 9 months) [ REF: https://wiki.ubuntu.com/LTS ] [ REF: https://wiki.ubuntu.com/SoftwareUpdates ]. All releases are classified as: development (progress towards the next final release), current (the most recent final release), supported (a previous final release that is still receiving updates) [ REF: https://launchpad.net/ubuntu/+series ][ REF: https://wiki.ubuntu.com/UsingDevelopmentReleases ]. In addition to the monthly and daily releases, there are pre-alphas, alphas, betas, and release candidates releases [ REF: https://wiki.ubuntu.com/Testing/ISO/Milestones ], while working towards a final release.