ERB (Embedded Ruby) is a templating system for Ruby. It allows you to embed Ruby code within text documents, typically HTML, XML, or YAML files. When the ERB file is processed, the Ruby code is executed, and its output is inserted into the document. This makes ERB a powerful tool for generating dynamic content, such as web pages that display data from a database or configuration files that are customized based on user input. ERB files are commonly used in Ruby on Rails web applications for creating views, which are the templates that define the structure and content of web pages. The Ruby code within an ERB file is typically enclosed in special delimiters, such as `<%= %> for outputting the result of an expression, <% %> for executing Ruby code without outputting anything, and <%# %>` for comments. ERB provides a flexible and efficient way to separate the presentation logic from the application logic in web development.