From Seo Wiki - Search Engine Optimization and Programming Languages
Edge Side Includes or ESI is a small markup language for edge level dynamic web content assembly. The purpose of ESI is to tackle the problem of web infrastructure scaling.
It is fairly common for websites to have generated content. It could be because of changing content like catalogs or forums, or because of personalization. This creates a problem for caching systems. To overcome this problem a group of companies (Akamai; Art Technology Group; BEA Systems; Circadence Corporation; Digitial Island, Inc.; Interwoven, Inc.; Open Market, whose ESI-related technology is now owned by FatWire Software; Oracle Corporation and Vignette Corporation) developed the ESI specification and submitted it to the W3C for approval.
ESI Language Specification 1.0 was submitted to the World Wide Web Consortium (W3C) for approval in August 2001. The W3C has acknowledged receipt, but has not accepted the proposal.
How ESI is implemented
ESI element tags are inserted into HTML content during creation. Instead of being displayed to viewers these ESI tags are directives that instruct an ESI processor to take some action. The XML based ESI tags indicate to the edge-side processing agent the action that needs to be taken to complete the page's assembly. One simple example of an ESI element is the include tag which is used to include content external to the page. An ESI include tag placed in-line within an HTML document would look like this:
<esi:include src="http://example.com/1.html" alt="http://bak.example.com/2.html" onerror="continue"/>