Any Bonsai tree which cannot be kept outdoors during frosty weather is considered an indoor bonsai. These are generally tropical and subtropical trees. Some indoor Bonsais can be left outdoors for brief periods of time during the warmer seasons, but it really depends on the tree and the condition it is in. Please, keep in mind; just because a tree is considered indoor does not mean it can thrive in any room of your home. The best place is probably going to be in the window which lets in the most amount of sun. There are many requirements you will have to meet for any Bonsai tree including, but not limited to the temperature and lighting. For this reason you should not buy a Bonsai tree spontaneously. Make sure you understand what the Bonsai tree you will be purchasing will need in advance so do not fail from the get go.