In Objective C, whats the difference between Instance and Class methods, and how do you switch between the two?

An instance method is a method that's only available on a particular instance of a class, only after said instance of the class has already been created. A class method is a method that can be called on the class itself, no instance necessary. For example:

YourClass *object = [[YourClass alloc] init];
[object someInstanceMethod];

The above is an instance method, because you must create an instance of YourClass (in this case, the object variable) before you can call the method "someInstanceMethod". However, in a class method, you don't have to create the variable first, and you can simply call the method directly on the class name, like this:

[YourClass someClassMethod];

Instance methods are the most common type of method, because you'll most commonly be telling a specific object to perform some action. Class methods can be useful, however, when there's no contextual data needed in the method, and it can theoretically be implemented and run on any generic object. Class methods are common in convenience initializers, though. For example, instead of typing this:

NSObject *newObject = [[NSObject alloc] init];

you can type this to produce the same result:

NSObject *newObject = [NSObject new];

That can be done on any NSObject or subclass thereof. In the above examples, both alloc and new are class methods, that both return a new instance of the NSObject class. init is an instance method, which sets up the new instance of NSObject that alloc returned and subsequently returns that. The new class method invokes both of those for you and returns the result.

Because of the difference in the way they work, class methods are declared a little differently than instance methods. Commonly, you'd declare and define an instance method like this:

@interface YourClass : NSObject
-(void)someInstanceMethod;
@end

#import "instance.h"
//There's some bad syntax highlighting by Treehouse's Markdown interpreter in this block. Ignore it, because the code works
@implementation YourClass
-(void)someInstanceMethod{
    //Whatever you want your instance method to do here
}
@end

However, with a class method, you'd declare and define your method with a "+", instead of a "-", like this:

@interface YourClass : NSObject
+(void)someClassMethod;
@end

#import "class.h"
//Also bad syntax highlighting to be ignored here
@implementation YourClass
+(void)someClassMethod{
    //Whatever you want your class method to do here, just like in an instance method
}
@end