Just started diving into Objective-C and IOS development and was wondering when and the correct location I should be declaring variables/properties. The main piece of code i need explaining is below:

Why and when should i be declaring variables inside the interface statement and why do they have the same variable with _ and then the same one as a property. And then in the implementation they do @synthesize tableView = _tableView (I understand what synthesize does)

Thanks :-)

@interface ViewController : UIViewController <UITableViewDataSource, UITableViewDelegate> 
{
    UITableView *_tableView;
    UIActivityIndicatorView *_activityIndicatorView;
    NSArray *_movies;
}

@property (nonatomic, retain) UITableView *tableView;
@property (nonatomic, retain) UIActivityIndicatorView *activityIndicatorView;
@property (nonatomic, retain) NSArray *movies;