Returns the count of characters (not bytes) in +self+:

  'foo'.length        # => 3
  'こんにちは'.length  # => 5

Contrast with String#bytesize:

  'foo'.bytesize        # => 3
  'こんにちは'.bytesize  # => 15

Related: see {Querying}[rdoc-ref:String@Querying].
