We had this discussion before numerous times, check the archive. To add a few more comments for your particular question:
A 'column' is the most general term. It refers to an article written by a 'columnist' expressing the columnist's personal opinion about something. Since it is opinion, it is not 'news', and therefore in most newspapers is not usually placed on the front page, or on other news pages. A column may be written by a newspaper employee or invited guest. It has the implication of some regularly scheduled appearance.
An 'editorial' is more specific, and is a column written by an employee of the newspaper, typically someone on the 'editorial board', and represents the opinion of the board, which usually reflects that of the owners of the paper.
'Op-ed' is a type of column written by someone not an employee of the newspaper; it may reflect the same opinion as the editorial board, or often an opposing one. The 'op' stands for 'opposite', and contrary to popular opinion does not mean an opposing viewpoint. The origin of 'Op-Ed' is from the New York Times in the 1970s which first published opinions, often opposing ones, on the page facing their editorial page, that is, they were directly opposite the editorials when you opened the paper up to the editorial page.
So 'Op' < 'opposite' < the page physically opposite the Editorial page.
For a longer description see related discussion