Some homes have problems with mold. There are lots of different kinds of mold, and most grow in damp areas of a home, usually away from light, heat and ventilation. Mold spreads by airborne spores, so it can show up anywhere growing conditions are right for it. Most of us can't tell one mold from another, and some are harmless. The problems come in when some people have an allergic reaction to the spores in the air. So if you or your home inspector sees something that looks like mold, it's best to have it removed, and not take any chances.
Mold remediation is the process of removing the mold. It can be washed off of hard surfaces, like concrete or aluminum siding. The moldy materials can be removed and destroyed, like moldy ceiling tile. It the case of mold in the attic, the surfaces can be blasted with carbon dioxide (dry ice), while the loose mold is vaccuumed up. Most of these processes are best left to professionals, because breathing the loose, dusty material may be hazardous to your health.
Equally important, you need to change the conditions that were good for growing mold, or it will come right back. Basements may need dehumidifiers, bathrooms may need better vent fans, attics may need better airflow or less humidity entering the attic space. It might be wise to seek professional help when figuring out what to change. You'll want to eliminate the chance that the mold will reoccur, while not wasting a lot of energy with the repairs.