instance methods use an instance of a class.

@interface MyTestClass : NSObject

- (void)testInstanceMethod;
    
@end

They could then be used like so:

MyTestClass *object = [[MyTestClass alloc] init];
[object testInstanceMethod];

Class method can be used with just the class name.

@interface MyClass : NSObject

+ (void)aClassMethod;

@end

They could then be used like so:

[MyClass aClassMethod];

class methods are the convenience methods on many Foundation classes like [NSString's +stringWithFormat:] or NSArray's +arrayWithArray