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The first number is the release year (8 == 2008) and the second is the release month (04 == April, 10 == October).

I really don't know what the third number stands for. At least they're not using it anymore.

For the LTS versions, there are "service packs" of sorts. Indicated by the last number. eg later this year you will see 12.04.1

.04 gets released in April (4th month) and .10 gets released in October (10th month)

* is then the last two digits of the year of release

So the version lets you know the year and month of release.

Edit: Ubuntu is on a 6 month regular release cycle, which is why they are all .04 and .10.

Also, the x.04 versions are LTS releases, every two years. 10.04 and 12.04 are both LTS.