The Portal Server has following services that cache data to improver performance. Take a look at WebSphere Portal 6.1 Performance Tunig Guide for detailed information on how to tune these services
The navigator service manages the content model for unauthenticated users, which controls the pages those users are able to see. This content model is periodically reloaded by WebSphere Portal; new pages which are visible to unauthenticated users will not be available until the next reload occurs.
This service also controls the HTTP cache-control headers which will be sent on unauthenticated pages
WebSphere Portal maintains information about many resource types in its databases. Some of these resources are replicated into memory for faster access; this is provided by the registry service. This replicated information will be periodically reloaded from the database, thus picking up any changes which may have been made on a peer node in a clustered environment.
The registry service holds information related to portlet defintion, portlet application definition, theme, skin, client.
Cache Manager Service
The Cache Manager Service is responsible for managing the different caches used in WebSphere Portal Version 6.1. It used to manage user Portal Access Control cache, Portal Environment Cache, Data Store, Portal User Management Cache. Take a look at WebSphere\wp_profile\PortalServer\config\CacheManagerService.properties file to find out what type of information is stored in this cache.
The portal provides two different types of caches: shared and non-shared.
The shared caches are cluster aware. This means that deleting an element from the cache on one cluster node results in deleting that element from the corresponding cache instances on all other nodes. This ensures that frequently changing data are kept consistent over the whole cluster installation.
The non-shared caches are used for data where cluster awareness is of no concern. This avoids unnecessary network communication overhead.