>Isn't the gram the basic unit (after all if it was the kg it couldn't very well be made up of a thousand of something else <g>)

Originally, yes.

The so-called "metric system" is not one system, but actually several systems, which of course share several characteristics.

In one of those, called "cgs", the mechanical units were based on centimeter, gram and second. Note that here cm. are used instead of m. IIRC, there have been more than one cgs-systems; the differences appear in units for electricity and magnetism, etc.

Modern systems tend to use a variation of mks (meter, kilogram, second). The reason is that in cgs-systems, units tend to be very small, so you have to use a lot of prefixes like mega, giga, etc.; or very large numbers.

(Note also that in both mks and cgs systems, the second is used for time, which was originally derived from the day: 1 second = 1/86400 day.)

The current system is called "SI" (Système Internationale), and is based on seven base units:

meter for length
second for time
kilogram for mass (NOT for weight)
ampere for electrical current
kelvin for temperature (NOT centigrade)
mol for quantity of substance (used a lot in chemistry)
and some unit for luminescense, which I forget.

>Wasn't the gram originally defined as the mass of 1cc of water ? Maybe not very accurate but at least it meant that you could potentially weigh things using a tape measure...

That is the original definition, but the current definition of the kg. is much more accurate - and now, a definition is sought which is not only accurate, but also reproducible.

The original definition is not as easy as it sounds (although the major problem seems to be the accuracy: how accurately can this be reproduced): 1 cc. of water - of what water? i.e., at what temperature, and of what isotopes should it be made up? 4 degrees Centigrade would be ideal, because water has its largest density at that temperature (and therefore, the rate-of-change with temperature is minimal). But I don't remember if this was specified. For the isotopes, probably the most common isotopes would be used. The ideal would be to specify all this exactly, so that the measurements are the same in other places of the Universe.