In Ruby, a method provides functionality to an Object. A class method provides functionality to a class itself, while an instance method provides functionality to one instance of a class.

Consider the following Ruby class:

class SayHello
  def self.from_the_class
    "Hello, from a class method"
  end

  def from_an_instance
    "Hello, from an instance method"
  end
end

This would yield the following:

>> SayHello.from_the_class
=> "Hello, from a class method"

>> SayHello.from_an_instance
=> undefined method `from_an_instance' for SayHello:Class


>> hello = SayHello.new
>> hello.from_the_class
=> undefined method `from_the_class' for #<SayHello:0x0000557920dac930>

>> hello.from_an_instance
=> "Hello, from an instance method"

We cannot call an instance method on the class itself, and we cannot directly call a class method on an instance.

Railstips has a nice article with more detail and a discussion of alternative ways of creating both class methods and instance methods.

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See also: Python: class vs. instance vs. static methods