Taken from the Microsoft.com Using CRM site:
You have to customize, or supervise customization of, Microsoft Dynamics CRM 3.0 and you do not have time to attend training or read a long book. You have opened Microsoft Dynamics CRM and are familiar with the basic features for Sales, Service, and Marketing. You want to make sure that you have a good, high-level understanding of the customization capabilities and are using the correct customization feature for the business requirements. You also want to be aware of common issues other users have experienced. With that goal in mind, these are the first six things to know about how to customize Microsoft Dynamics CRM.
This series of articles describes some basic customization features and concepts. These articles highlight key information about the capabilities and limitations of these feature areas.
1. Meta-data driven customizations – Considerations for customizing Microsoft Dynamics CRM.
2. Security and customization – Describes the privileges you must have to access Customization features and how security applies to the customizations that you create.
3. Form and view customization – The most common types of customizations. What you can do and the process to apply changes.
4. Personal customizations – Let users personalize Microsoft Dynamics CRM to meet their work styles.
5. Entity customization – Explains what an entity is and how you can customize and relate entities.
6. Transportable customizations – How to move your customizations between Microsoft Dynamics CRM servers.