#import instead of @class

[ravi_14]

#import "Person.h"   
@class Asset;                  //i have used import "Asset.h" which contains definition for asset 
@interface Employee : Person
{
    int employeeID;
    NSMutableArray *assets;
}
@property int employeeID;
- (void)addAssetsObject:(Asset *)a;
- (unsigned int)valueOfAssets;
@end

I got this snippet from objective c book.
In the second line , author has used @class.
I have implemented with import "Asset.h"
Please explain the difference between two and common scenarion where we prefer @class.
Many Thanks.