Wool Slabs, as the name implies, are slab versions of their wool block counterparts. A fun use could be as a very thick carpet, or as carpeted stairs.
Wool Slabs are created like any other slab, by putting three wool blocks of one color in a row in the crafting grid. However, if you use more than one color, then White Wool Slabs will be crafted.
Wool Slabs can be crafted back into wool blocks by putting two in a crafting grid. Once again, the color rule still applies. This can be used to un-dye wool, if you need to.