What is the difference between an abstract and an introduction?
Open any scholarly article and it will have an abstract followed by the actual paper starting with an introduction.
I would not say the two are similar or even close. An introduction is a lead in into the issue, it presents background knowledge, explains why the issue is relevant/topical/problematic/etc., actually introduces the reader to the topic of an essay demonstrating its place and relevance. It finishes with a thesis statement stating the argument the paper makes.
An abstract is close to the summary, but it is rather a summary of the paper than of the argument. For example, imagine you have a paper reviewing healthy benefits of physical activity and giving relevant recommendations. In the abstract, you do not repeat all the benefits or all the recommendations but tell (1) what is the main message/conclusion of the paper and (2) what type of information can be found in the paper, and (3) (not necessary) who might be interested in this paper. For example,
The paper is concerned with the effects of physical activity on human health. In particular, it presents a review of literature providing ample evidence of the exceptional role of physical activity in the promotion of health and wellbeing and prevention of diseases. Further, prevalence and barriers to engagement in physical activity are discussed. Therefore, the paper gives recommendations on how to tackle physical inactivity to improve the health of the nation and cut healthcare costs. The paper will be of interest to health care professionals and policy makers.