这个问题的答案来自于stackoverflow，老外人家回答问题就是细心、认真，学习一下。
A：Can anyone explain the difference between use and require, both when used directly and as:use and :require in the ns macro?
B：require loads libs (that aren't already loaded), use does the same plus it refers to their namespaces with clojure.core/refer (so you also get the possibility of using :exclude etc like withclojure.core/refer). Both are recommended for use in ns rather than directly.
A：If I require lib foo, then to use bar in foo, I'd have to write foo/bar every time, right? Why would you want to load a lib in ns but then not refer it into the ns? I guess you might be worried about collisions, and you don't want to bother having to reconcile them, right?
B：not having to reconcile collisions is a good point, and more generally there's a programming style which says "namespaces are a honking great idea, we should have more of them" (from "The Zen of Python") -- so e.g. that style recommends not using "using namespace foo;" in C++, so that readers and maintainers of the code won't have to worry "where does this bar come from" but see a more explicit foo::bar instead. require (vs use) supports this "explicit namespaces" style.
总结一下：
use就是在当前项目中引用别的项目时，通过clojure.core/refer把其他项目的函数变量等引入当前项目的工作空间中来，这样在引用其他项目中的函数时就不用再写项目名了。
require比use功能少些，不把别的项目引入当前工作空间，在引用别的项目的函数时需要写全项目名+函数名。
至于其中的原因，老外说的很清楚了。that style recommends not using "using namespace foo;" in C++, so that readers and maintainers of the code won't have to worry
扩展阅读：
http://stackoverflow.com/questions/3408076/difference-in-clojure-between-use-and-require
http://stackoverflow.com/questions/871997/difference-between-use-and-require
