Wood mulch, whether wood chips or wood bark, is made from many kinds of trees, but primarily pine and cypress. More often than not, bark mulches are made from pine trees. Some mixed-mulch types are just labelled "hardwood."
Parts of the Tree
Wood chip mulch is taken from the wood beneath the bark. Often, these chips are from young, farm-raised trees, or chopped-up leftover pieces from a lumber yard. Bark mulch is just that: mulch created from the bark of the tree. This, too, is often obtained from lumber yards or woodworking facilities, where the bark must be shaved away from the wood before the wood is worked.
Bark mulch tends to be darker and full of large, flat sections of rough wood, or nugget-shaped sections. Wood chip mulch may contain various sizes of wood chips accompanied by pieces of bark or even leaves, and it varies in colour depending on the tree from which it was taken: from blond to reddish to light brown.
Properties as Mulch
Bark mulch floats away in heavy rain, but doesn't decompose rapidly. The North Carolina State University Extension notes that bark mulch tends to be used more for soil conditioner, while the nuggets are used as mulch. Wood chip mulch varies so much in composition that it may last quite a while or may need seasonal refreshing. It doesn't float away as easily as bark, and may even form a mat.
Greenhouse gardens or other areas where there's no threat of the mulch washing away benefit most from pine bark. Wood chip mulches vary greatly, and should be selected based on their source (some mulches are not harvested ecologically), the environmental conditions of the garden, and the aesthetic they provide to the garden as a whole.