AWS's Well-Architected Framework is a set of best practices for designing and operating reliable, secure, high-performing serverless applications. The framework consists of six pillars: security, reliability, performance, cost optimization, sustainability, and operational excellence. To build secure serverless applications, developers can adopt best practices such as limiting Lambda privileges, implementing virtual private clouds, managing secrets and credentials, and using secret management services like AWS Secrets Manager. Reliability is achieved by ensuring high availability, managing failures, and optimizing function memory size. Performance optimization involves reducing cold starts, implementing caching, reducing initialization times, and optimizing function memory size. Cost optimization can be achieved by right-sizing Provisioned Concurrency and using Application Auto Scaling. Sustainability is ensured by defining and enforcing sustainability SLAs, running workloads in Availability Zones that use renewable energy, removing unused components, and scheduling jobs to prevent resource contention. Operational excellence is achieved through centralized monitoring with a platform like Datadog, which provides full visibility into the serverless application's health and performance.