Thymeleaf is a Java template engine that can be used as a view technology in Spring applications. It integrates well with Spring and allows templates to be defined using HTML/XML syntax with additional Thymeleaf attributes and expressions. Templates can include static elements as well as dynamic content through standard dialect expressions. Templates can also be extended and customized through additional dialects. Thymeleaf provides advantages such as support for multiple template modes, internationalization, caching, and extensibility, though it can be slower than some other template engines.