what is the difference between butter spread and pure butter?
Usually things titled butter spread are actually made from largely vegetable oil.  They can be a replacement for butter in recipes where the fact that there is fat in the recipe is all that matters (quick breads, brownies, muffins, etc.).  In general if you have a recipe where melted butter joins the wet ingredients and those wet ingredients get mixed into the dry ingredients, you should be able to get by using butter spreads (at least the oil-based kinds).

On the other hand, butter spreads aren't going to work well in baking that requires the creaming method (where butter and sugar are creamed together) because the texture is "firm out of the refrigerator, softens quickly" (source) as one spread maker says.  The bubbles needed to make baked goods rise using the creaming method won't hold.