Their argument goes that since human genetics have scarcely changed since the stone age an ideal (for health) diet would be a reconstructed stone age diet. Therefore through studying archeology and modern hunter gatherers we can learn what a healthy diet looks like.
Foods which are not edible raw and unprocessed are excluded from the diet. These include grains, beans and potatoes. Foods which are included in the diet are meat, fish, fruit, vegetables, nuts and eggs. The single exception to this rule is dairy. Dairy is excluded despite being edible raw since it is nevertheless a post agricultural food. Some closely related diets such as that recommended by the Weston A Price Foundation http://www.westonaprice.org/ follow similar principles but do include dairy.
The non-animal foods available on the diet are the same as those available in raw veganism. However there are two fundamental differences between raw veganism and the paleodiet. Firstly paleodieters consume meat and other animal products (in fact usually more is consumed than on a standard modern diet, in some cases substantially more). Secondly any and all food can be cooked if desired.
Normally the diet is what would be considered low carb though not to the extent of say the Atkins Diet. Unlike Atkins fruit and vegetables are consumed in large amounts. The vitamin and mineral content of the diet is very high compared to a standard diet.