NOTE: Please take this info with a pinch of salt. It is only my experience, and not a definative guide. I take no responsibility for others use of this information.
OK, Here is the info off the top of my head.
First and foremost, there are a few different types of suits.
Single layer - One outer Layer of fire proof material
Dual Layer - Two Layers of fire proof material, One outer, one inner
Triple Layer - Three layers of fire proof material, One outer, two inner.
Single layer is suitable for club events, upto and including motorkhanas, hillclimbs, sprints.
Dual Layer is suitable for anything in the National level of motorsport - Rallys, targa's and the above mentioned stuff - Not including things like V8 supercars.
Triple Layer required for any FIA stuff and international rally's.
There are two standards that are recognised by CAMS and FIA. And that is the 1986 FIA standard and the 2000 FIA standard.
Without this approval on the neck of the suit the suit cannot be used in the events of national and state level.
Apparently the 1986 standard is being phased out, but CAMS has not yet put a due by date, so you may find that most shops are selling these suits at reduced prices.
Next, Suit Material.
There is from what I see, Wool & Nomex.
Wool is a natural fibre and as such has some different properties to that of Nomex, which is a man made material. If you want to find out what they are......google it.
As for which is better, Nomex is more expensive, looks shineier (is that a word?) and feels nice (too me) on.
Wool is less expensive by almost about a third, looks more dull and can be (but not always) itchy on the skin.
For go-karting (racing) you need an abrasive resistant suit, which is made from a polymer like nylon. These suits CANNOT be used in car racing due to the non compliance with the fireproof material requirements.
So for my case a single layer suit will be suitable since I will not be racing go karts, only mucking around in them.
so, there it is.......
A very simple guide/layman explanation of race suits how what where.