Not to be confused with...
'article' or 'opinion column' or 'essay' ... an 'article' is intended to present detailed and interesting information about a defined subject to a general audience; and the information should be explained in a clear, fair and reasonably objective way (the journalist has a 'duty to inform'). An 'opinion column' will usually be focused on one particular issue, which should be explained convincingly and your final opinion argued reasonably, even if it is a very subjective point of view. An 'essay' is supposed to be a methodical and objective review of arguments, based on solid factual evidence, and reaching a logical conclusion (which is not the same as just a quick opinion).
In addition, both 'opinion column' and 'essay' are likely to have more defined audiences - 'opinion column' = the audience of the publication in which it appears; 'essay' = thoughtful, educated people, rather than just anybody.