I want to know the meaning of the letters in the project navigator in Xcode; (Ms & A)
These are for source control. M stands for Modified. A stands for Added. This is so you know what files have changed since the last commit of your project.
It's all explained on this support page http://developer.apple.com/library/mac/documentation/ToolsLanguages/Conceptual/Xcode4UserGuide/SCM/SCM.html