Targetpuncher gave you some very good info. It is confusing.

The only part he left out was, when it comes to the 30.06, the .06 stands for the year of it's development, 1906. It was originally designed and designated as the 30.03 in 1903, but after cartridge and rifle improvements were made, they redesignated it to 30.06. I'm sure there was some other cartridges designated this way too. So you see, it can be very confusing as to the second set of numbers. 30-30, 30.06, 25.06 and then like he said, all the "European" designations.