we use NSDocumentDirectory mostly to save files which are used in the project. but we have two other paths in the iPhone application,  they are also read and writable paths as documents directory. now lets see the difference between all three

NSDocumentDirectory:
it is permanent storage(exists until the app will be deleted)
and all files which are in this directory are sharable in iTunes

NSArray *docs = NSSearchPathForDirectoriesInDomains(NSDocumentDirectory, NSUserDomainMask, YES);
NSString *documents = [docs objectAtIndex:0];

NSLibraryDirectory:
it is permanent storage (exists until the app will be deleted)
and files in this directory will not be shared in iTunes.

NSArray *lib = NSSearchPathForDirectoriesInDomains(NSLibraryDirectory, NSUserDomainMask, YES);
NSString *library = [lib objectAtIndex:0];

NSCachesDirectory:
this is temporary storage i.e, files will be deleted by iphone(i think these files will be deleted if app is terminated) and files in this direcotry will not be sharable in iTunes. its better to store temperary files

NSArray *cach = NSSearchPathForDirectoriesInDomains(NSCachesDirectory, NSUserDomainMask, YES);
NSString *cache = [cache objectAtIndex:0];