The biggest difference between strong and weak study skills lies in the application.
For example, a student may know on Monday that they need to get ready for a test that is on Friday. They may know that they should start early, they may know what the material is that they need to learn, and they may know the format of the test. But, if they do not know HOW to apply this knowledge to prepare for they assessment then they have only achieved organization, not "study skills". Organization is important skill in studying but isn't a study skill in itself .
In order to move from the level of weak skills to the level of strong skills, the student must know what to do with this information. This may mean that on Monday they make a list of how they will prepare each day leading up to Friday and then follow through with that plan. This may mean that they take the material that they need to learn and analyze it for the best way to internalize the material, such as writing notes on reading and reviewing those notes, making and studying flash cards, or teaching the material to a classmate to synthesize and verbalize the information. They may also study in terms of what they know about the final test, such as creating their own multiple choice questions and answers, write out the answers to possible essay questions, or making True/False flash cards.
Strong study skills also involve the ability to blend and synthesize the different information they are given about an assessment and come up with a comprehensive study plan that involves several of these study methods as well as incorporates what they know about the ways they study best.
The above is a thorough example of the differences between strong and weak study skills but by no means is it a definitive explanation of the only types of strong and weak study skills. This example can be applied to just about any situation to describe the difference between week and strong study skills as long as the difference that is emphasized is that strong study skills require application of information while weak study skills are usually limited only to organization.