Vert.x is an open source framework by the Eclipse Foundation to build reactive and polyglot software. It's reactive because it uses asynchronous streams to respond to any changes or events.

It's polyglot because it supports different programming languages based also on the JVM, such as Java, Groovy, Ruby, Python, and JavaScript.

Vert.x, like its competitor Node.js, uses an event bus (technically a loop) to interact with components of the application, passing events to appropriate handlers to manage them asynchronously.

Vert.x comes with few concepts that are necessary to explain.