Why This Is a Good Question
This is a tough question to Google for unless you know the right search terms. The #{} operator technically performs expression substitution inside a string literal.
The Answer
The #{} literal is the operator used for interpolation inside double-quoted strings the same way that the backticks or $() construct would be used in Bash. From a practical point of view, the expression inside the literal is evaluated, and then the entire #{} expression (including both the operator and the expression it contains) is replaced in situ with the result.
Related Links
- http://www.ruby-doc.org/docs/ruby-doc-bundle/Manual/man-1.4/syntax.html#string
- http://www.ruby-doc.org/docs/ProgrammingRuby/html/language.html
- http://en.wikibooks.org/wiki/Ruby_Programming/Syntax/Literals#Interpolation
- Ruby (on Rails) syntax