When you say abs, this means that you are referring to well defined abdominal muscles that you may have on your abdominal area. This is something that a lot of men and women are working on because they want to show that they are taking good care of their bodies. It will take discipline and hard work before people are able to get what they want.
When you say six-pack, this means that you are referring to the protruding muscles that can be found over the abdominal area. There are different exercises that people normally have to do to achieve the type of abdominal muscles that they want. Some would just like abs but some would really like to get a six-pack.