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Most units used inside ROS are SI units (http://en.wikipedia.org/wiki/International_System_of_Units). Although you can't rely on it, that's usually a good assumption. Have a look at the message descriptions. Sometimes you find useful information there.

So, in your case position/translation is measured in meters and rotation in radians. If you see a rotation description with x, y, z, w, than that's a quaternion.

If your measurement data (e.g. odom) doesn't align with your real world measurements, than your software and/or hardware introduces these errors. For example, the Create's/TurtleBot 1's odometry is very unreliable.

Hope this helps!