what's the difference between "abstract" and "introduction"?

This afternoon, one of my students asked me a question about the graduate theses: what's the difference between "abstract" and "introduction"?

It's a common and complex question for the beginners, esp. for the undergraduates.

Here, we can find some suggestions for this question:

At first glance, it might seem that the "introduction" and the "abstract" are very similar because they both present the research problem and objectives as well as briefly reviewing methodology, main findings and main conclusions.  However, there are important differences between the two:

Introduction:

an introduction leads the reader into the subject of study by giving the state of art of knowledge in the area, the lacunae, and reasons for undertaking the present study.

it should be short, but does not have a word limit;

Main purpose is to introduce the research by presenting its context or background. Introductions usually go from general to specific, introducing the research problem and how it will be investigated.

Abstract:

An abstract is a sort of summary of the entire paper. It should be specific about the main findings and not vague. It will also specify what was the aims and goals of the study. so,

It has a maximum word limit;

It is a summary of the whole research;

Its main purpose is to summarize the research--particularly the objective and the main finding/conclusion, Not to introduce the research area.