One of the main problems with using chemotherapy is that it is a broad acting therapy - it does not only target cancer cells. This is why the side effects of chemotherapy are broad ranging and affect many areas of the body. This limits the dose people are able to tolerate.
Targeted therapies are more efficient in their action as they will only act on cells which contain specific cell markers, and will not act on other cells without the markers.
Each of our body's cells contain thousands of markers that identify that the cells belong to you and what function they perform. Our immune system use these markers. When certain white blood cells come into contact with other cells, they look for markers that identify if the cell belongs to you or not. If the cell has the markers that identify that they belong to you, the white blood cell moves on. If the markers on the cell do not belong to you, the white blood cell will attempt to destroy it, as it is likely a foreign pathogen (eg. like a bacteria) that could do you harm.
Target therapies work with a similar principle. Each targeted therapy has a specific cell marker that it can target. If a cell has the marker, the therapy is able to affect that cell. If a cell does not have the specific marker, the therapy will not affect it. This ensures the therapy is targeted to the problem cells only.
Targeting treatments may be able to reduce the general side effect profile seen in the more generalised chemotherapies.