Values consists in general of three different parts. The quantity, the unit and the datum they are part of. The most common is the SI-system, where the last revision is of 2006 and next is due 2014-2015. Other similar systems are the CGS and MKS systems. In some cases, notably for old values, there are no formal datum. Over time the datum can change, due to updates or drift in artifacts they relie upon. For example was the length measurement meter redefined in the SI system in 1960, and there are draft changes to the definition of the weight measurement.
It is important to note that the use of a specific datum is often implied in the use of the unit without being said explicitly. A "meter" usually refer to the SI-system as of the latests revision. When it comes to to values with less rigid definitions, or no formal definitions at all, it can be unclear what is meant when only the unit is given. For example will it be difficult to have a clear understanding of what "feet" refer to without further information.
If the datum is well-defined (also implicitly) and the unit given it is often possible to convert between units with only modest loss of precision. It will for example be possible to convert between imperial feet and meter. It will not in general be possible to convert between an unspecific "feet" and "meter" or beween "coffestops" and "meter".