Evil Spirits, Shaytan (satan) or Jinn is Allah creature created from fire. It is invisible to the human. They dwell in places such as cemeteries, wastelands, unclean places like bathrooms and un-inhabited homes. They can posses human bodies unintentionally or through black magic make do things beyond our control, making ones life miserable.
Evil spirits can possess a human when one is not clean; Care is not taken, while urinating or defecating; going to cemeteries and wastelands without saying prayers and being at places where there is lots of negative energy such as funeral or sadness.
Evil spirits are powerful but not as powerful as Allah. Therefore, when proper care is taken to recite Allah name and the recommended prayers, one can be away from from evil spirits and if one is possessed, one can be rid of such evil by Allah's will.