What is the difference between Microsoft Certification 20331 and 70331 Exam. Please let me also know which is the basic Certification Exam for SharePoint 2013.
As of today (December 30, 2014):
If you're interested in pursuing a Microsoft certification for working with SharePoint, the certification you should take depends on if you're primarily an IT Pro or Developer: If you're an IT Pro, you're likely interested in the MCSE - SharePoint Certification: https://www.microsoft.com/learning/en-us/mcse-sharepoint-certification.aspx
If you're a developer, you'll find the MCSD - SharePoint Applications Certification more applicable: https://www.microsoft.com/learning/en-us/mcsd-sharepoint-apps-certification.aspx
Both of these certifications target SharePoint 2013/SharePoint Online.
20331 is the formal course available that covers exam material in 70-331 -- that information is detailed on the two certification pages linked above.
The MCSE and MCSD are both professional level certifications and can be quite difficult if you don't have the suggested experience with each technology (typically 5 years). If you're looking for entry-level Microsoft Certifications you can check out the Microsoft Technology Associate Certification tracks: https://www.microsoft.com/learning/en-us/mta-certification.aspx
Do some more searching around on the web and you'll find plenty of material and preparation tips floating around.