Creating pages based on for example user agent information, IP address, or referring page for the sole purpose to fool the visitor. Sometimes used to feed spiders, like Googlebot, with pages optimised for the search engine or other service they belong to. This is called "cloaking". Often used to give the browser a page which uses the abilities of that browser to the maximum. Since most of this information can be invalid, wrong, or faked I recommend against this. Design your pages in such a way that they work with as many browsers as possible. This strongly reduces maintenance time.
Also allowing only some countries to view your pages (or excluding countries) can fail for example when visitors use anonymizing proxies.