Citing is where you indicate within the body of the text the author and the year the work was published. You should also include the page number in your citation if you are quoting from a particular page, or using ideas from a particular page.
The reference comes at the end of your essay/ assignment/ dissertation, where you give the full bibliographic details of the information, so that someone reading your work can find or trace the source for themselves. Your reference list should be in alphabetical order, i.e. in alphabetical order of author surname (or title, if you cannot find the author's name).