Blend Shapes are used to morph an object into a different on. It's like parenting but only changing the objects apperance throuh vertices.
This is helpful when lip syncing a model, giving it eye brow movement and adding a streachyness to the mouth when opened.
Blend shapes can also save time remodeling the eye brow on the other side of the face by duplicating the main head twice and an altered head, example an eybrow that's risen. Rename one of the heads base and the other inverese and name the risen eye brow head bshape(blend shape). Flip the inverse head and get it roughly in the same position as the base head. Next you need to wrap them by selecting base then inverse and apply the wrap deformer. Next apply a blend shape by selecting the bshape head then the inverse head. Finally bring up the blend shape animation editor window and set the influence as high as it can then select the base head and delete its history. Sounds long but it is not, this will save time resculpting the other side of the face.