Every object and class exposes several methods. Class methods are methods that can be invoked directly on classes, for example ActiveRecord's finders:
author = Author.find_by_last_name("Ginsberg")
Similarly, there are instance methods that can be invoked on any object of a given class, like the downcase method of a String, or to keep in line with ActiveRecord's example:
books = author.books
Then there are the so-called singleton methods, which are a special type of method that can be defined, and which exist only for a specific instance of a class:
my_string.my_method # my_string has a singleton method my_string2.my_method # Raises a NoMethodError
Ruby doesn't really differentiate between the three of them when you call a given method on a class or an object. The interpreter only cares about determining whether or not the receiver exposes that method. When you use the syntax receiver.method, Ruby is actually sending a message with the name of the method, and its arguments, to the receiver object (be it an object, class, or module). As a matter of fact, the following two are equivalent, and the dot notation is just sugar syntax for the developer:
"antonio".capitalize # "Antonio" "antonio".send(:capitalize) # "Antonio"
You can see the advantages of the dot notation when chaining multiple method calls together as shown here:
puts "$32.90".sub('$','Đ') # Prints Đ32.90 Kernel.send(:puts, "$32.90".send(:sub, '$', 'Đ')) # Prints Đ32.90
As a reminder, the values ...