Everything you wanted to know about all the different ways to measure time (systems of time) including:
- Universal Time (UT, UT0, UT1)
- Coordinated Universal Time (UTC)
- International Atomic Time (TAI)
Follow the "leap second" link on the page for a discussion of how these time systems drift from each other, and how they're corrected. The link also discusses some other units of time like Ephemeris Time (ET).
And if you're curious, Greenwich Mean Time (GMT) was based on the solar mean day.