WebLogic is an Application Server. To be more precise, it is a
J2EE Application Server. The latest version of WebLogic fully implements the J2EE 1.3 Specification, which includes the
Servlet,
JSP, EJB, and JMS APIs (among others).
Why is it so popular? WebLogic (formerly known as Tengah) was one of the first
Java Application Servers, so it had a headstart. Secondly, it is a good product and has a large developer base. This makes it very attractive to large corporations.