Mind explaining a bit? Asking out of genuine curiosity/ignorance on the subject.
Well, I can get you as far as Caucasians are named such because the Caucuses(sp?) area about the middle of the big Caucasian umbrella.
You've got the Indo-Europeans on one side, scoot through the near and middle east into the Persia-Northern India. The Aryan peoples, for instance, are what most people would consider native to the Indian Sub-continent, and are Caucasian. Indo-Iranian, is Iran, the Kurds and the Ajam peoples, for instance, even look like Indo-Europeans to the point you could pick one up, dress them appropriately, and drop them in any city in the US or Canada and they'd probably pass without issue.
I'm not up on the genetics of the situation, but the linguistic research shows a root 'Indo' language that would be the primordial precursor to everything from English to Iranian. There are also a lot of mythological connections between the Nordic Pantheon and the Hindu one. So a lot of scientists believe, on those grounds, that it's all part of the same major ethnic grouping.
This is the 'Sub-Saharan African' umbrella for Caucasians. About the same land overall I think, except the Semitic-Arabic ethnic groups are kinda there in the middle.
Well... not to be too much of a pedant, but some things are getting jumbled here (or explained in a way that makes it seem jumbled).
To explain "Caucasoid", you need to understand that humanity is primarily divided into three main racial groups, which are (quite inexactly) named "Negroid", "Mongoloid" and "Caucasoid". Or: black people, yellow people and white people. These all have sub-groups, and there are many "mixed" racial groups living in the regions where these primary groupings blend into each other geographically.
Caucasoid is
not the same as Indo-European. In fact, the Semitic peoples mentioned above are
also Caucasoid. And the Basques (who are remnants of Europe's pre-Indo-European population) are likewise Caucasoid.
The Indo-Europeans, both linguistically and genetically, are
not originally an "Indo" group. That hypothesis has been very thoroughly debunked, and prevails strongly among Indian ultra-nationalists alone. The Indo-Europeans came from the Pontic Steppe, where they domesticated the horse (indeed,
all domesticated horses on Earth today descend from
one stallion they domesticated). This was their "super-weapon", and using it, these horse-riders (in an age where all other peoples got by on foot) conquered most of Eurasia. They settled Europe, they settled Anatolia, they settled greater Iran, they settled the North of India, they settled Central Asia (where the 'Stans now are) and they settled the Tarim basin.
Since then, the Tarim basin, most of Central Asia and all of Anatolia have been taken over by Turkic people. (Those are originally Mongoloids, but in many places, the Turks have ethnically mixed with the populations they conquered, so that they are quite often generically half Indo-European or whatnot.)