Car.h

//
//  Car.h
//

#import <Foundation/Foundation.h>

@interface Car : NSObject

@property (nonatomic, assign) NSInteger horsepower;

+ (Car *)car;
- (void)vroom;

@end

Car.m

//
//  Car.m
//

#import "Car.h"

// C method. It returns nothing--`void`.
// Within these methods we can call public
// methods declared on the `Car` class.
void makeCarGoVroom(Car *car) {
    [car vroom];
}

// Another C method.
void makeCarThatThenGoesVroom() {
    Car *car = [Car car];
    makeCarGoVroom(car);
}

@implementation Car

// Instance method.
- (void)vroom {
    // I can access instance variables such as _horsepower.
    // Here, `self` refers to this object, an instance of the `Car` class.
    for (int i = 0; i < self.horsepower; i++) {
        NSLog(@"Vroom!");
    }
}

// Class method.
+ (id)car {
    // Cannot access instance variables such as _horsepower.
    // Here, `self` refers to the `Car` class.
    return [[self alloc] init];
}

@end