NSSet是什么对象， 怎么用

//NSSet
//Unordered collection of objects.
//Immutable. You cannot add or remove objects to it once //it’s created.
//Important methods:
+ setWithObjects:(id)firstObj, ...;  // nil terminated
- (int)count;
- (BOOL)containsObject:(id)anObject;
- (id)anyObject;
- (void)makeObjectsPerformSelector:(SEL)aSelector;
- (id)member:(id)anObject; // uses isEqual: and returns a matching object (if any)

//NSMutableSet
//Mutable version of NSSet.
+ (NSMutableSet *)set;
- (void)addObject:(id)anObject;
- (void)removeObject:(id)anObject;
- (void)removeAllObjects;
- (void)unionSet:(NSSet *)otherSet;
- (void)minusSet:(NSSet *)otherSet;
- (void)intersectSet:(NSSet *)otherSet;