Corrective Action: Also known as a CA, Corrective Actions are actions taken to eliminate the root cause of an existing nonconformance and to prevent its reoccurrence.
Preventive Action: Actions taken to eliminate a 'potential root cause' to a potential problem that has not occurred.
ISO 9001:2000 clauses 8.5.2 and 8.5.3 require that you identify areas for both within your organization. The CA portion deals with items such as customer complaints received, nonconformances uncovered as part of internal quality audits, or received incoming materials not meeting specifications for example. These problems exist in the here and now and require corrective action to prevent their reoccurrence. What do you do to correct the issues and how do you document what you did? Did it work (close the loop)? Do you review this as part of Management Review?
Preventive Action is a more difficult idea to grasp for most (maybe because we tend to put out fires rather than prevent them) and can be accomplished in various ways. I like to think of it this way. If a process or system is in a state of statistical control, does a problem exist? Sometimes but not always. There is natural variation in any process or system, but their output may not meet our desired expectations. A process not meeting our expectations may need refining, but not necessarily correcting. We often think something 'must' be wrong when we don't get the results we hoped for. There is a significant difference in a process not meeting expectations and a process not producing to specification. This misunderstanding of system output usually leads to a landslide of issuing Corrective Action for every problem encountered. Does your organization make defective products occasionally? Mine does. Should I create a Corrective Action Report for each defective product especially if some level of defects is to be expected (the system will never produce zero defects)? No. Doing so would bog down the CA process and creates another set of problems (missing CA reports, CA reports not filled correctly, duplicate CAR for the same problem). Not all problems are Corrective Action opportunities. Some are Preventive Action opportunities. This example is more typical of a Continuous Improvement process with Preventive Action ties. Give thought to which situation you have and take actions commensurate with risks encountered. I hope this helped.
For a 'definitive' discussion, see: Corrective Action vs. Preventive (Predictive) Action - A Definitive Discussion