In total, there are 5 PMI certifications (listed in order of difficulty):
CAPM (Certified Assistant in Project Management): The easiest PMI certification, aimed at beginners in project management. Does not make one a project manager, but it's good for entering the PM territory.
PMI-SP (Project Management Institute: Scheduling Professional): A PMI certification focusing on the scheduling aspect of project management, has less requirements (and prestige) than the PMP.
PMI-RMP (Project Management Institute - Risk Management Professional): Another PMI certification that focuses on one aspect of project management, and in this case it's Risk management. Requirements are not as steep as those for the PMP, but they are focused on the risk management experience of the applicant.
PMP (Project Management Professional): By the most important project management certification in the world. Currently there are 400k+ PMP certified project managers.
PgMP (Program Management Professional): The most important program management certification. Extremely hard to get, the applicant goes through 3 difficult steps in order to earn it.
All the above certifications, with the exception of CAPM, need to be maintained through earning PDUs every 3 years (30 for the PMI-SP and the PMI-RMP, 60 for the PMP and the PgMP). If you want to maintain your CAPM, you have to retake the exam every 5 years.