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A category adds methods to an existing class — even one you don't own — without subclassing.

#import <Foundation/Foundation.h>

@interface NSString (Reversed)
- (NSString *)reversed;
@end

@implementation NSString (Reversed)
- (NSString *)reversed {
    NSMutableString *result = [NSMutableString stringWithCapacity:self.length];
    for (NSInteger i = self.length - 1; i >= 0; i--) {
        [result appendFormat:@"%C", [self characterAtIndex:i]];
    }
    return result;
}
@end

// Now ANY NSString has -reversed:
NSString *s = @"hello";
NSString *rev = [s reversed];      // "olleh"

The (Reversed) after the class name is the category name — purely descriptive.

Use cases:

• Add convenience methods to UIKit/Foundation classes
• Group related methods of a large class across files
• Provide platform-specific implementations

Caveats:

• Can't add instance variables in a category (only methods)
• If two categories define the same method, behavior is undefined
• Method calls go through dynamic dispatch (no compile-time check across categories)

Class extensions — anonymous categories (), used inside the class's own .m file to declare "private" methods/properties:

@interface MyClass ()       // class extension — no name
@property NSInteger internalCount;
- (void)privateHelper;
@end

Anything declared in a class extension is part of the class itself — but only visible in that file.