There are many types of Stupas (Chortan in Tibetan) some are incredably detailed and some are simply,, simple.

The stupa represents the mind of enlightenment. They are used to house precious artifacts of Buddhism such as relics, thangkas, statues, and mantras. In a sense they are far better than having any singular statue in your home because they incompass all and every Buddha, Bodhisattva, and Deity. The only item better than a stupa are scripture of "direct transmission" which are usually called "Jewels". The online store, tibetan treasures, has one of these jewels in there store, this was written by Longchen Rabjam.... Filling Stupas is a unique  and old practice, by filling the Stupa you are offering to the Buddhas perfected and sublime enlightenment, you invite the Deities into your space where they continue to pay homage to the Buddha, and in doing so bless you and all your surroundings. You can get the fillings for statues and Stupas at "Patola Gate" another online store, you can get very special and powerful mantras for your Stupa at FPMT (foundation store) these are so powerful that offering a small bell to your Buddha or Stupa with these inside will bless all who hear that bell from then on. That is just one of many, many other blessings that come from having these mantras. You can find some very strong and well made Stupas at "Nirvana - Industeries". Injoy!