From the ESE website:
ESE uses Google to search the contents of more than 23,000 economics web sites. They come from RFE, home pages reported by economists in RePec Author Services, andEDIRC. A Google Custom Search Engine searches these pages. As this technology is still in beta testing, the results might not be ideal. In particular, Google uses an approximation algorithm to search these sites.
ESE offers a “Search Plugin” for Internet Explorer 7.0 or FireFox 2.0 and 3.0 users. It allows you to initiate searches directly from a search box in your browser. You should be offered one for ESE when you check your search plugins.