The fields in which I've written papers (Computer Science, Psychology and Theater Science) the abstract is a compressed version of the whole article. Think one or at most two sentences each of introduction, method, result, and discussion.
The abstract exists as a kind of sample of the paper and should summarize the main/most important points in each of the above sections. Its goal is to give a reader a quick introduction into the article and in at most one page, preferrable even less, give them an idea of if they should read it or not. Most of your readers will be other students/researchers with a specific field of research or even an article of their own to write.
The introduction ... introduces the context and previous research pertaining to the topic of the article.
If you think of your article as an hourglass where the introduction gives a broader view of the topic and then the method and result focuses that broad view on the exact subject you've chosen, and then the discussion goes wide again, discussing implications of your results, connecting them to a broader context, etc.
This is how I was taught to write papers, however, this differs widely. Mostly, I think, because fields and the people in them are different, and many different levels of article quality are allowed to pass through.
For instance, psychology is extremely scientific. I can't recall a single paper that didn't follow the same format and they all had a very high quality. Especially papers on experiments. Probably because experiments fit perfectly into the intro-method-result-discussion format. (And psychology is all about the experiments!)
Computer science on the other hand sometimes felt like coder high-school, at least when you compared to psychology.
Theater science... Oh my... I wrote a first-year paper, and only in critique (is that what it's called when other students critique it?) I finally understood that I'd actually not fully understood what I was doing... (And yes, I was sober the whole time!)