Since Oracle acquired Sun Microsystems in 2010, Java Certification's Names have changed.
Sun Microsystsems used to have two exams. The SCJP (Sun Certified Java Programmer) was meant for programmers and the SCJA (Sun Certified Associate) was meant for those who wanted broader knowledge.
These names were change by Oracle to be OCJA and OCJP. With Java version 7, Oracle decided to shut down the OCJA and split the OCJP in two certification levels.
The first level is called OCAJP (Oracle Certified Associate), or also 1Z0-808 (free 1Z0-808 questions available here), and the professional level is called OCPJP (Oracle Certified Professional), or also 1Z1-809 (free 1Z0-809 questions available here).
Oracle also provide the option to upgrade an old certification to JAVA SE 8 with the "Upgrade to java SE 8 Programmer" certification, also known as 1Z0-810.
Recently, Oracle also released the Java Foundations Certified Junior Associate (1Z0-811). This certification is more intended for managers or other people that needs to understand the programming domain but are not actually involved in coding.
A good view of the Java Certification world has been provided at JavaCro2015 by Hujak and it is reported in the image below. You can visit the original presentation visiting Java Cro 2015 presentation.
The most important thing to remember is that you do not have to book OCAJP before OCPJP, however you will not receive any certificate until both exams are passed.
Oracle Java Certified Associate - Java SE 8 Programmer (1Z0-808)
This is usually the first step for achieving your Java Programmer Certification. Since Oracle updated the Java Certifications, even this exam requires a deep preparation.
You will be tested on topics like Java Data Types, Java operators and decision constructs, arrays, loops, encapsulation, inheritance and a set of selected class from the Java Api.
Oracle Java Certified Professional - Java SE 8 Programmer (1Z0-809)
This is usually the second and last step for achieving your Java Programmer Certification. You have more chances to pass this exam if you use Java in your day by day working life.
You will be tested in topics like Java class design, Generics and Collection, Lambda Built-in Functional Interfaces, Java Stream API, Exceptions and assertions.
Oracle Certified Professional - Upgrade to Java SE 8 Programmer (1Z0-810)
This exam is for those people that own a previous Java Certification and want to update it to JAVA8.
You will be tested on topics like Lambda Expressions, Using Built in Lambda Types, Filtering Collections with Lambdas, Collection Operations with Lambda,