反向链接又称外部链接，指被链接的网址，比如我的博客链接到您的某个网站，那么这个链接就是您网站的反向链接。
为什么反向链接对网站被搜索引擎收录很重要呢？让我们从搜索引擎的原理来讲起。
搜索引擎通常派出一个能够在网上发现新网页并抓取文件的程序，这个程序就是我们通常所说的蜘蛛，搜索引擎蜘蛛从数据库中已经知道的网页开始出发，访问这些文件并抓取文件。并且搜索引擎蜘蛛会跟踪网上的链接，访问更多的网页，就像现实用户跟踪链接一样，只有通过这样我们才能够发现新的网址，同样当蜘蛛发现有新网址时，将会把新网址记录入数据库等待抓取。
可见，搜索引擎要能收录一个网址，前提是找到该网址，就如我们发现好的网站，肯定是通过反向链接找到的。没有反向链接，搜索引擎连页面都发现不了，一切的优化都是白费。也就是说要想收录就其基础就得有反向链接。
不过，考虑到能更快、更好的收录到网页，搜索引擎通常会提供一个收录入口。在收录入口提交您的网址（没有被收录过的新域名），类似于蜘蛛发现您的网址并记录入数据库。
百度的收录入口：http://www.baidu.com/search/url_submit.html
google的收录入口:http://www.google.com/addurl/
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