Annotations in Java are a form of syntactic metadata that provide additional information about the program without affecting its execution. They can be applied to various program elements such as classes, methods, variables, and parameters. Annotations serve multiple purposes, including aiding the compiler in error detection, enhancing code readability, and facilitating code generation during compile-time or deployment. Java provides a set of predefined annotations, such as @Override and @Deprecated, while also allowing developers to create custom annotations. Understanding annotations is essential for leveraging frameworks like Spring and JPA, which utilize them for configuration and code generation.