Machu Picchu has been declared as One of the Seven wonders of the world in 2007 at Lisbon, Spain.
Boleto - Not access to every where
Yes - Boleto will not give you access to the most famous sites in Cusco city. So if you plan to visit only the city, don't buy it, as is almost useless. So check first what your plan is, then check the places it has permission to enter.
If you are planning to visit places in and around Cusco, you may buy Boleto. It is the admission ticket for some of the tourists spots in Cusco. It is also known as BTG (Boleto Turístico General). But before you buy this ticket, you need to check the list of sites you can enter with this tickets, as some this ticket does not include some important sites of Cusco.
Museo de Arte Religioso
Museo Historico Regional
Museo de Artey Monasterio de Santa Catalina
Museo Palacio Municipal
Museo de Sitio Qoricancha
Iglesia San Blas
It is not costly -$10 for adults and $5 for students under age 26 with ISIC card. Don't forget that this ticket is valid for only ten days (which should be enough, as you may not stay that long in Cusco).
Where to buy ..
It is supposed to be available in the sites where you can enter with it. But practically in most cases they don't have it. So it is better to buy this tickets from one of the following tour offices in Cusco.