Houseplants are plants which are growing indoors in pots. They can help to make a room more pleasant, as well as freshen the air by producing oxygen and even to some extent help "clean" the air. As well as growing them for decoration, "house plants" could include herbs, fruits (e.g. dwarf citrus), and vegetables which are also grown for consumption.
Indoor environments are often very different to outdoor environments in most parts of the world, so the range of practical houseplants are often very different to typical regional outdoor plants.