说明文是用言简意赅的语言来介绍客观事物，讲清事理，并给人增加知识的一种文体。说明文解说事物的特点、发展变化规律，分析前因后果，目的在于使读者“明白事理”，获得必要的信息，对事物有个较完整明晰的了解和认识。说明文不同于记叙文和议论文，它重在给人介绍，让人明白，因而，轻“情义”重事实。说明文与议论文关系密切，有人将其统称为议论文。但在写作的目的和方法上两者仍有区别。说明文主要是通过事实客观地解释或说明事物本身；而议论文则以事实、数据等为证据，表明主观见解，使读者信服或赞同某一观点。

历年四级考试中的“How I Overcome Difficulties in Learning English”、“My Ideal Job”以及图表作文的第一段都属于说明文的范畴，如，2002年6月的根据坐标图写题为“Students Use of Computers”的作文。

说明文必须按一定的顺序进行说明：时间顺序、空间顺序、逻辑顺序或认识顺序，选择具体的特点和细节来说明。

所以，说明文的思路要围绕说明顺序、事物的特点和细节展开。例如：

范文点评

A Famous Clock

When you visit London, one of the first things you will see is Big Ben, the famous clock which can be heard all over the world on the B.B.C. If the Houses of Parliament had not been burned down in 1834, the great clock would never have been erected. Big Ben takes its name from Sir Benjamin Hall who was responsible for the making of the clock when the new Houses of Parliament were being built. It is not only of immense size, but is extremely accurate as well. Officials from Greenwich Observatory have the clock checked twice a day. On the B.B.C. you can hear the clock when it is actually striking because microphones are connected to the clock tower. Big Ben has rarely gone wrong. Once, however, it failed to give the correct time. A painter who had been working on the tower hung a pot of paint on one of the hands and slowed it down!

【点评】这是《新概念英语》中的一篇经典说明文，介绍了世界著名的“大本钟”。文章开篇指明该钟位于英国伦敦，且全世界的人们通过BBC就能够听到钟声。接下来讲述了“大本钟”的由来，特点——不仅外型巨大，而且走时准确，在结尾处讲了“大本钟”的一个幽默的小插曲——油漆工把油漆桶挂在指针上，把钟弄慢了。细读这篇文章，我们不难发现，区区159个单词就把“大本钟”的光辉形象牢牢地刻在读者的脑海里，为什么？本文基于事实，分析了主题的来龙去脉，语言简洁又不失生动贴切，作者态度客观又充满幽默，使读者既“明白了事理”，又对事物有较完整明晰的了解认识，此乃说明文之真谛也。