What is ERB in Rails?

ERB (Embedded Ruby) is Rails' default templating engine. It lets you embed Ruby code inside HTML files using special tags: <%= expression %> evaluates Ruby and outputs the result (HTML-escaped), <% code %> executes Ruby without output, and <%- -%> suppresses whitespace. View files use the .html.erb extension. ERB is compiled to Ruby at runtime. Alternatives include Haml (indentation-based, concise) and Slim (even more concise). Rails also supports Jbuilder and json for API responses.