Ok while looking through the stuff for DS...I notice there is v6....then v6 HD. Then lilith and lilith HD....etc

I looked at the promo it looks better, more detail but...is it a texture? or mesh? Can you use textures on them? as they do not seem to come with maps (maybe I am wrong).

Are they displacement maps?

and most importantly...

are they necessary? O.0

Are they necessary?  Only if you want more detail in your figures.  I believe they are morphs since they show up on the shaping dial, rather than surfaces, but I'm not an expert on that and there are others around that are.  There is also some debate about how much detail certain ones add to your render, they aren't all the same.  They can add quite a bit of realism to places like elbows, knees, and abdomens for close up work where those areas are visible.

Can you live without them?  Sure.  I'd pick up at least the V6 one though.  Having one for each character gets to be expensive.

o.0 I am even more confused now. I thought those characters were all v6 just different texture maps and morphs. Like ones I have for v4. I can use one morph but different maps....cant do that with v6 hd?

I am a user of the program like you, and not a PA, so my knowledge of the background workings is limited.  I have been using DAZ since 2010 though and one does pick up useful tidbits of information along the way.  All HD are morphs, and the morphs tend to follow the morph of the character they are based on, you can dial that basic V6 morph into about any female character.  In the beginning the HD character was sold independently from the basic morphed character, I imagine people got tired of paying for the same character twice, and later ones have the base and HD characters combined.  HD may subdivide the mesh further, I'll need someone with more knowledge than me to clear that up for you.  In any case, you can dial in or out HD morphs anytime you want, they do add to rendering time.

Here is an excerpt from on HD character:

"Liu Yan is a remake of the popular Summer Edition Liu character. Her skin has been completely re-textured to fit Victoria 6 perfectly. She comes along with a custom HD morph for V6's body and face, as well as a bonus HD morph for Mei Lin."

If you want a complete listing of characters with HD morphs, just search for "HD" (at DAZ anyway, doing that here will not pick up anything useful because for a vendor to use DAZ's HD tools it has to be sold at DAZ).

Since HD is morphs not textures, you can use any of your existing textures on HD Genesis 2 characters if you can use them on regular Genesis 2 characters.

AH ok so the HD is actual figure not just a displacement map like I would use in poser. AH ok then...

Basically it's a system that allows you to create morphs using a SubD version of the mesh, and then apply it to the base resolution mesh, which if you tried to do without this system would result in a failed morph.

It is a morph that only shows up under the correct level of Sub-D or higher. With 3Delight the morph shows up because 3Delight always automatically determines the correct level of SubD based on various factors. In other render engines (Iray, Lux, Octane, Firefly, etc.) you have to dial the correct level of SubD to see the difference. (Note that in Iray it is Render time SubD level all others it is Viewport subD level.)