Oracle web cache is component of 10g Application Server which is installed by default with Portal & Wireless or BI & Forms type Installation. You can also install standalone webcache for J2ee & webcache applications or to configure webcache with
Why I need Web Cache ?
As name suggest web cache act as Cache for Web Servers including Oracle Application Server 9i, 10g to cache frequently used static & dynamic pages. Web Cache is used for increasing response time of your site by caching frequently used partial / full page , images, pdf. Web cache can also act as Software Load balancer or Reverse Proxy Server or Front end to your Applications to Hide server name.
Is Web Cache configurable & Certified with Apps 11i ?
Yes, Web cache can be configured with Apps 11i as reverse proxy server (for security) and Failover & Surge Protection (To minimize downtime) .
*Web Cache can't be used as Load balancer for E-Business Suite as its not possible to set session stickyness in Webcache. (Thanks to Gaurav for pointing this out)
Check Metalink Note 306653.1 Installing and Configuring Web Cache 10.1.2 and Oracle E-Business Suite 11i
How to start / stop webcache ?
To start/stop webcache with 10g AS, use
opmnctl startprocstopproc ias-component=WebCache
For standalone J2ee & Webcache Installation use
How to troubleshoot Webcache Issues ?
Webcache logs are at $ORACLE_HOME/logs event_logs & access_log , all issues errors are recorded in event_logs so check this file any issues related to Webcache .