从微观角度将电子参考源分为数据库、传统工具书的电子版、搜索引擎、电子期刊等类型，并对各类型电子参考源作详细论述。

Divide electronic reference resources into various types such as database, electronic reference book, search engine, electronic journal from micro-angle and describe the types in detail.

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为了克服搜索引擎的这种缺点，产生了针对某一专门需要，用于信息提取、信息过滤和异构信息源整合的元搜索引擎。

In order to overcome these problems meta search engines aiming at special requirements appear for information extraction, information filtering and integration of heterogenous information resources.

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