I will try to keep the Oracle certification jargon as simple as possible here.
First question might be, why would one go for Oracle certification ! Is it useful for career ! how relevant it is ?
My Answer to this is if you work in this technology, going for certification will add to your knowledge and weight of your resume.
Getting Oracle certified places you ahead in your career and its one of the prestigious certifications in the software industry, given by one of the world's leading corporation, i.e. Oracle Corp., and is the most trusted and used database product in the world as it has lots of features and is very powerful in terms of security, scalability, availability and stability.
That's why I believe Oracle certification is worthy to go for. You can do more research on your own to reach a conclusion. Lets move ahead...
Oracle has several products under its portfolio; few to name are like Oracle Database, Oracle Application server, Oracle Reports, Oracle Forms, Java etc.
It provides different levels of certifications in various products.
I have done certifications in Oracle DBA area and PL/SQL development area, so will mention about both of them here.
Do you want to become an Oracle DBA (Database Administrator) or PL/SQL programmer (Database Application Development) ? If you have decided that, it will be easy to choose a certification path.
I have written an article about how to choose among DBA or programmer path, click here for more info.
So once you have decided to become a DBA, you can go for becoming an OCA (Oracle Certified Associate) first, then OCP (Oracle Certified Professional) and later an OCM (Oracle Certified Master).
You need to be an OCA first to go for OCP certification, and an OCP first before going for OCM certification.
For becoming OCA, you need to clear some exam(s). Which exams? that is decided by Oracle Corp. The exams keep on expiring, changing or updating in terms of their validity and syllabus.
Oracle exams have exam codes like 1Z0-007, 1Z0-031 etc.
As an example, for becoming Oracle 9i OCA, I had to clear 2 exams, 1Z0-007 (Introduction to Oracle 9i) and 1Z0-031 (Oracle 9i Database: Fundamentals I).
For Oracle 11g OCA, I had to clear exam 1Z0-052 (Oracle Database 11g: Administration I) and as I already had completed 1Z0-007, I became 11g OCA. So I had to give 1 exam for OCP.
For becoming Oracle 11g OCP, I had to clear exam 1Z0-053 (Oracle Database 11g: Administration II) and complete one of the many hands-on courses.
OCM certification requires good expertise in the area. You need to be an OCP and need to clear a 2 day long performance based exam, conducted on workstations under close supervision of examiner.
OCA and OCP exams are objective-type based. OCM is different.
For different database versions, there will be different exams for each with their own exam codes.
So when you want to go for any Oracle related certification, just search 'Oracle certifications' in your search engine and explore the certification menu there. You will find which exam is needed for respective certification type.
Database Application Development or PL/SQL Programmer path
If you want to do certification in SQL or PL/SQL, you can go for OCA and OCP certifications in PL/SQL and "SQL Certified expert" certification for SQL.
Again, you need to clear 2 exams for PL/SQL OCA. I cleared exam 1Z0-144 (Oracle Database 11g: Program with PL/SQL) and as I already had completed 1Z0-007, I became 11g PL/SQL Developer Certified Associate (i.e. OCA in development path).
For OCP, you need to be an OCA and then you need to clear once more advanced PL/SQL exam. That's it, you will be OCP in this path. There is no OCM in the PL/SQL path.
So when you want to go for such exam, Google search 'Oracle certifications' and explore the certification menu there.
Next: Check my article on How to prepare for Oracle certification exam, some tips.