Part of Canonical’s predictable versioning scheme includes an expectation of how long any given release will be supported. Support here means updates to the various software packages, increasingly just critical security fixes as versions reach the end of their projected span. The company behind Unbuntu has two different time scales when it comes to that span of providing updates.
Ubuntu 8.04 (released in April of 2008) is a long time support release which means Canonical has committed to keeping it patched for three years. The H Security notes that the clock is about to expire for this release, code named Hardy Heron. If you have any aging systems still using this ultra stable release, it is time to finally upgrade it to a newer LTS release. Come May, Heron will no longer receive updates.
Ubuntu Desktop 8.04 LTS approaches end of life, The H Security
Regular, non-LTS releases are supported for 18 months, half as long as the long term cycle. Canonical has another version, according to Linux Journal, 9.10 that is come up on its end of life. According to the article, the recommended upgrade target is the current LTS version, 10.4.
Canonical To Drop Support For Ubuntu 9.10, Linux Journal
Since Ubuntu hits a new version every 6 months, it is possible to stay ahead of obsolescence without always having to run the latest and greatest which for many software packages, let alone full Linux distros, often require a bit of shakedown before proving trustworthy.
And, of course, the LTS server editions are supported for five years.