Introduction. What's the difference between 介绍, 简介, 入门, 引导, 导语, 导论, 引子? I've seen 介绍, 简介 and 导语 in news pieces, which labels a one sentence synopsis of the article. Is there any difference between these three? 介绍, 简介 look to be interchangeable, and used commonly like 介绍 person/thing/concept.

What about 入门, 引导, 导论, 引子? Looking at Baidu hits, they all seem to be used similarly.

Baidu hits:

介绍 100m

简介 100m 

导语 38m

入门 100m

引导 100m

导论 28m

引子 18m
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