Your home has two types of lines for plumbing, the water lines that bring water in and the sewer lines that take wastewater out. This keeps water separated and allows for the wastewater to be disposed of ina proper and sanitary manner. When there is a clog further into the sewer line, however, it can cause this sewer water to back up into the house which is not only a nasty sight but a nasty smell. Clogs like this can form from large amounts of toilet paper, hair, or grease. If you notice a foul odor and some backup you may want to consider calling a professional.