Introduction. What's the difference between 介绍, 简介, 入门, 引导, 导语, 导论, 引子?
简介 is actually a short form of 简单介绍, a brief introduction, while 介绍 is introduction.  In this way they are interchangable just as they are in english  

导语 - 'lead', i.e. the few words you will see at the beginning of a novel.
入门 - 'getting started', i.e. Macbook for dummies = Macbook 入门
you get the idea for the rest:
引导/引子 - 'guide'
导论 - a more detailed introduction