There are three core issues recognized for WAN-based distributed Web crawling systems: Web Partition, Agent collaboration and Agent deployment. Centering around these issues, this paper presents a comprehensive overview of the current strategies adopted by academic and business communities. The experiences, problems and challenges encountered by the WAN-based distributed Web crawlers are classified and discussed in depth. A summary of the current evaluation indicators is also given. Finally, conclusion and some suggestions for future research are put forward.