Is there a difference between an "instance variable" and a "property" in Objective-c?

A property is a more abstract concept. An instance variable is literally just a storage slot, like a slot in a struct. Normally other objects are never supposed to access them directly. A property, on the other hand, is an attribute of your object that can be accessed (it sounds vague and it's supposed to). Usually a property will return or set an instance variable, but it could use data from several or none at all. For example:

@interface Person : NSObject {
    NSString *name;
}

    @property(copy) NSString *name;
    @property(copy) NSString *firstName;
    @property(copy) NSString *lastName;
@end

@implementation Person
    @synthesize name;

    - (NSString *)firstName {
        [[name componentsSeparatedByString:@" "] objectAtIndex:0];
    }
    - (NSString *)lastName {
        [[name componentsSeparatedByString:@" "] lastObject];
    }
    - (NSString *)setFirstName:(NSString *)newName {
        NSArray *nameArray = [name componentsSeparatedByString:@" "];
        NSArray *newNameArray [[NSArray arrayWithObjects:newName, nil] arrayByAddingObjectsFromArray:[nameArray subarrayWithRange:NSMakeRange(1, [nameArray size]-1)]];
        self.name = [newNameArray componentsJoinedByString:@" "];
    }
    - (NSString *)setLastName:(NSString *)newName {
        NSArray *nameArray = [name componentsSeparatedByString:@" "];
        NSArray *newNameArray [[nameArray subarrayWithRange:NSMakeRange(0, [nameArray size]-2)] arrayByAddingObjectsFromArray:[NSArray arrayWithObjects:newName, nil]];
        self.name = [newNameArray componentsJoinedByString:@" "];
    }
@end

(Note: The above code is buggy in that it assumes the name already exists and has at least two components (e.g. "Bill Gates" rather than just "Gates"). I felt that fixing those assumptions would make the actual point of the code less clear, so I'm just pointing it out here so nobody innocently repeats those mistakes.)

A property is a friendly way of implementing a getter/setter for some value, with additional useful features and syntax. A property can be backed by an instance variable, but you can also define the getter/setter to do something a bit more dynamic, e.g. you might define a lowerCase property on a string which dynamically creates the result rather than returning the value of some member variable.

Here's an example:

// === In your .h ===

@interface MyObject {
    NSString *propertyName;

}

// ...

@property (nonatomic, retain) NSString *propertyName;

// === In your .m @implementation ===

@synthesize propertyName /* = otherVarName */;

The @property line defines a property called propertyName of type NSString *. This can be get/set using the following syntax:

myObject.propertyName = @"Hello World!";
NSLog("Value: %@", myObject.propertyName);

When you assign to or read from myObject.propertyName you are really calling setter/getter methods on the object.

The @synthesize line tells the compiler to generate these getter/setters for you, using the member variable with the same name of the property to store the value (or otherVarName if you use the syntax in comments).

Along with @synthesize you can still override one of the getter/setters by defining your own. The naming convention for these methods is setPropertyName: for the setter and propertyName (or getPropertyName, not standard) for the getter. The other will still be generated for you.

In your @property line you can define a number of attributes in parens for the property that can automate things like thread-safety and memory management. By default a property is atomic meaning the compiler will wrap @synthesized get/set calls with appropriate locks to prevent concurrency issues. You can specify the nonatomic attribute to disable this (for example on the iPhone you want to default most properties to nonatomic).

There are 3 attribute values that control memory management for any @synthesized setters. The first is retain which will automatically send release to old values of the property, and retain to the new values. This is very useful.

The second is copy which will make a copy of any values passed in rather than retaining them. It is good practice to use copy for NSString because a caller could pass in an NSMutableString and change it out from under you. copy will make a new copy of the input which only you have access to.

The third is assign which does a straight pointer assign without calling retain/release on the old or new object.

Lastly you can also use the readonly attribute to disable the setter for the property.

I use properties for the interface part - where the object interfaces with other objects and instance variables are stuff that you need inside your class - nobody but you is supposed to see and manipulate those.