Oracle's Certified Professional Java SE Programmer (OCPJP) exam is the fundamental exam required to demonstrate solid understanding of Java and some of its SE APIs and is a prerequisite to a number of the other Java certificates.
It is designed as a fairly detailed test of knowledge of the core features and constructs of the Java programming language. It tests a wide range of Java's APIs and core features, starting from basics such as looping constructs and variables, to more complex topics such as Threads, Collections and Generics.
It does not cover specific technology domains such as GUI creation, Web or network programming, though it does cover part of the APIs included in the standard library.
The exam tests how well a programmer has understood the language constructs and mechanisms. However it's not a goal of the exam to test the programmer's ability to produce purposeful or efficient programs.
It does not test the programmer's ability to write efficient algorithms, for example, though it does test knowledge of which collections should be selected in order to implement efficient algorithms without re-inventing the wheel.