Are you referring to the edges of the wood from a sawmill?
If so, there's no difference. Just a lot more bark. But it's a lot cheaper.

Slab wood is normally what's cut off the round portions of the trunk, which leaves a square core, or primal, from which boards can be cut.

As Christopher said, there's just a lot more bark.

I have a friend whose parents have heated their home with slabwood from a local sawmill for nearly 20 years. A half-tractor trailer sized stake bed truck loaded to the gills with slabs was, about 10 years ago, costing them under $200. A similar load of cord wood probably would cost triple that.

Of course they have to cut and stack it, and it's green when they get it, but they have a 2 or 3 years supply laid in, and a load normally lasts them most of the year.

slab wood is thinner and you will be loading the fireplace or woodstove more often..most of the bark comes off after it is dried and makes excellent kindling. The thickness goes according to how much the sawmill has to cut off to get to the inner wood to be cut for lumber...some thick, some thin, depends on alot of things, but usually they cut it thin for less waste.. we burned tons of slab wood when we lived in Missouri as we had access to local sawmills. We got it in 8' lengths usually( if it didnt break in sections) and cut it to fit the fireplace and woodstove..you stack it the same as cord wood and you burn it the same..only difference is size and how much work you want to put into it.

One good thing about slab wood that I forgot to mention is that it seasons very quickly because the pieces are so thin.

You could conceivably get a load of green slab wood in the spring or early summer and use it for heating that year.