Here is a list of some of the common validation helpers in Ruby on Rails;
validates_acceptance_of is used to see if someone has checked a checkbox during a form submission, the attribute does not need to have an associated database column.
validates_associated is used to see if the objects you are associating with your model are valid objects.
validates_confirmation_of is used when confirming things like passwords the convention is that you would confirm the attribute password against the attribute password_confirm.
validates_each is used to validate attributes against a block of code.
validates_exclusion_of is used to validate if an attribute is not in a particular enumerable object.
validates_format_of is used to validate the format of an attribute. This is handled with a regular expression and can be used to make sure a pattern does not exist within the attribute.
validates_inclusion_of is used basically as the opposite of exclusion, it makes sure the attribute is found within an enumerable object.
validates_length_of is used to verify that the length of the attribute matches the restrictions that have been put in place around it.
validates_numericality_of is used to ensure something is a number and can go as far to say a number greater than or less than something, or force it to only be an integer.
validates_presence_of is used to ensure that the attribute exists and is not nil when it is being saved.
validates_uniqueness_of is used to ensure that there are no other attributes with the same information in the system.
I hope this helps someone as an uber quick reference.