Vert.x is a toolkit for building reactive applications on the Java Virtual Machine (JVM). It is used by Malmberg, an educational publisher in the Netherlands, to build scalable e-learning applications using microservices. Vert.x is event-driven, non-blocking, and supports multiple programming languages. It allows building distributed applications where verticles (components) communicate through an event bus. Vert.x is well-suited for microservices due to its lightweight verticles that can be developed, deployed, and scaled independently.