Built-in roles are provided by Azure to cover the most common needs for access permissions, such as Owner, Contributor, and Reader. These roles are designed by Azure and cannot be changed. In contrast, custom roles can be created by users to grant specific permissions not covered by built-in roles. Custom roles are flexible and can be tailored to an organization's specific needs, allowing precise control over access to Azure resources. This distinction is critical in creating a security environment that both protects resources and provides sufficient access for users to perform their jobs effectively.