Another post for the newbies! This issue came up on the forums and I remember being confused by all the options and new terms out there too. Hope this helps to make it a little clearer.
In human terms, the dome is kind of like the top of the skull. It's the hard bit on top of her head under the scalp. As you might know, Blythe comes in different facemolds and these different kinds of Blythe have different kinds of dome. The older girls (BL, EBL and SBL) have the dome attached to the faceplate of the doll. This means that if you want to open her for customising or change her hair, you need to remove the scalp first. It's not an easy process, but that's a whole other post for another day.
Here's an EBL dome.
On the newer Blythes like RBL and FBL Takara/CWC have kindly made the dome a separate piece and attached the scalp to that. This means that opening these molds is much much easier (you don't need to soak or saw) and results in less damage to the doll. It's also really easy to swap scalps with another Blythe of the same mold in your collection.
Here's what an RBL dome looks like. The scalp is on this one as I am too lazy to pull it out again, but you can see the hard plastic dome on the inside.
Sometimes you can buy RBL and FBL Blythe without a dome - be warned, they can be difficult to source and without a dome, your Blythe's scalp can dent and well...look wierd.
The scalp is a rubbery covering with hair rooted in it that covers the dome. Removed from the dome, it looks like this (on the underside anyway...the other side is considerably more attractive!).
The little sticky out lip on it is called the flange. This helps it to sit nicely between the dome and faceplate. This scalp is an RBL Saffy Encore.
You can also buy blank scalps from Coolcat that look like this. Obviously, this isn't my pic. Don't sue me Coolcat! I think your scalps are great :)
They are used for rerooting a new head of hair when a stock scalp is not available. They are a litte different from the stock scalps in that they are made of thicker rubber and obviously, don't have holes in them yet. They only come in one colour, so might not match your Blythe's faceplate very well if she has dark skin. They are great for mohair reroots, scalpless Blythes or if you simply can't bring yourself to cut the hair off a stock scalp to reroot it.
Unlike most BJD's a Blythe's wig is not really her hair (that's her scalp). In some ways a Blythe wig is just like a human wig - it's a fun way of having a new look, but you can always tell it's a wig and it has limited styling options. A wig is not made of rooted plugs like a scalp, it's made by stitching wefts of hair to a stretchy or net base. If you move the hair the wrong way, you can see the wefts.
Here's Poe in her wig. This photo was taken after a bit of primping and from a carefully chosen angle. It is sitting directly on her dome as she does not have a scalp and it does not sit right. I have heard of people using them directly over a blank scalp either glued or velro'd on, but you have to remember that your styling options will be limited.
Wigs are best used over the top of a scalp, with the hair tucked up in a wig net. The nets are very inexpensive and most of the places that sell the wigs also sell the nets. A word of warning though, expect to have to wash your Blythe's hair after using a wig - especially if she has bangs. Her hair will be very messy when you take it out of the wig cap and her bangs will be all over the place.
A re-root is exactly that - re-rooting new hair into a scalp. Unlike a wig, this is more permanent and it takes quite a while to do. Blythes are rerooted to repair badly executed haircuts, to give them a new kind of hair, like mohair or to give them a unique new colour that isn't available on the stock scalps. The best way is to use the Blythe's own stock scalp to ensure a great colour match between face and scalp - also I have heard that it is much easier to use the holes from the factory hair, rather than having to punch your own. If you don't have your own Blythe's stock scalp, sometimes clean ones or ones with ruined hair come up for sale. You can also use a blank Coolcat scalp and punch your own holes in it. I will be doing one on my bait Blythe soon and will probably cover the process more thoroughly then.
Hope you found this post informative. I will be doing a follow-up on all the different kinds of hair in the near future.