WebLogic Server is an application server that provides a distributed architecture for deploying Java applications. It introduces concepts like domains, which define groups of servers, and administration consoles for managing servers. Applications are deployed as JAR, WAR, and EAR files and can be load balanced across server clusters. Performance tuning is important and involves configuring connection pools, thread management, and monitoring for issues like hangs, crashes and out of memory errors. WebLogic Server architectures have evolved from versions 8 through 10.