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[DocD83]

SI means "System Internationale" or however those crazy French would spell it. The International System to us base English-speakers.

ETA: I found a couple differences...the second is abbreviated as "s" rather than "sec" in SI, and there are no volume units (such as the liter), just cubes of length units. Also, my brother tells me the metric meter is based off of a master platinum rod whereas the SI meter is based off of the wavelength of a certain light.

And there is yet a THIRD system:

Scientists use a metric system of unit measures in their work. They use either the newer Système International (SI), which is the Meter-Kilogram-Second-Ampère (MKSA) system, or the older Gaussian system, which is the Centimeter-Gram-Second (CGS) system. The basis of both systems is metric and was created in France two centuries ago and refined, since then, to include electrical units.

[Lissa]

There isn't a whole lot of difference between meters-kg-s and CGS, when you get right down to it. It's a matter of scale- CGS works much better for benchtop, when measuring pinches of powder in kilograms seems silly

[DocD83]

Oh yeah hadn't thought of that.

[PoolMan]

DocD83 said:

Also, my brother tells me the metric meter is based off of a master platinum rod whereas the SI meter is based off of the wavelength of a certain light.

The metric system is partially based on the platinum kilogram, yes. Not completely, but it's the mass standard. Of course, ironically enough, the original kilogram is decaying (at a very slow rate), so it's becoming increasingly innaccurate.

D'oh.

No idea about the SI wavelength standard.

[DocD83]

Once again, researching my own questions has proved more interesting than doing my urgent homework.

From NIST:

The meter (m) is the Si unit of length and is defined as the length of the path traveled by light in vacuum during the time interval of 1/299 792 458 of a second.[ 3 ] This replaces the two previous definitions of the meter: the original adopted by CGPM in 1889 based on a platinum-iridium prototype bar, and a definition adopted in 1960 based on a krypton­86 radiation from an electrical discharge lamp.