A std::set is a container class in the C++ Standard Template Library (STL) that stores a collection of unique elements in a sorted order. It is implemented as a balanced binary search tree, which allows for efficient insertion, removal, and lookup of elements.

A std::multiset is similar to std::set but it allows duplicate elements to be stored.

Here is an example of creating and using a std::set and std::multiset:

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#include <set>
#include <iostream>

int main() {
    // Create a set of integers
    std::set<int> mySet;

    // Insert the numbers 1-5 into the set
    mySet.insert(3);
    mySet.insert(1);
    mySet.insert(5);
    mySet.insert(2);
    mySet.insert(4);

    // Print the set
    for (auto i : mySet) {
        std::cout << i << " ";
    }
    std::cout << std::endl;

    // Check if the set contains the number 3
    if (mySet.count(3)) {
        std::cout << "The set contains the number 3." << std::endl;
    } else {
        std::cout << "The set does not contain the number 3." << std::endl;
    }

    // Remove the number 3 from the set
    mySet.erase(3);

    // Print the set
    for (auto i : mySet) {
        std::cout << i << " ";
    }
    std::cout << std::endl;

    // Create a multiset of integers
    std::multiset<int> myMultiset;

    // Insert the numbers 1-5 into the multiset
    myMultiset.insert(3);
    myMultiset.insert(1);
    myMultiset.insert(5);
    myMultiset.insert(2);
    myMultiset.insert(4);
    myMultiset.insert(3);

    // Print the multiset
    for (auto i : myMultiset) {
        std::cout << i << " ";
    }
    std::cout << std::endl;

    // Check if the multiset contains the number 3
    if (myMultiset.count(3)) {
        std::cout << "The multiset contains the number 3." << std::endl;
    } else {
        std::cout << "The multiset does not contain the number 3." << std::endl;
    }

    // Remove all occurrences of the number 3 from the multiset
    myMultiset.erase(3);

    // Print the multiset
    for (auto i : myMultiset) {
        std::cout << i << " ";
    }
    std::cout << std::endl;

    return 0;
}
This program creates a std::set called mySet and std::multiset called myMultiset. 
It then uses the insert() function to insert the numbers 1-5 into the mySet. After
that it will print the set using range based for loop and it will check if the set contains the number 3 using the count() function. Then it will remove the number 3 from the set using the erase() function. After that it will print the set again.

Then it will insert the numbers 1-5 into the myMultiset and it will insert the number 3 twice, so there will be two occurrences of the number 3 in the multiset. After that it will print the multiset using range based for loop and it will check if the multiset contains the number 3 using the count() function. Then it will remove all occurrences of the number 3 from the multiset using the erase() function. After that it will print the multiset again.

As we can see, the main difference between std::set and std::multiset is that std::set stores unique elements, whereas std::multiset allows for duplicate elements to be stored.