chiral center or stereo center or stereogenic center are the names given to an atom

connected to four different atoms or four different Subsituent  groups

 i m not sure but chiral centre is any atom bonded to 4 diff. atoms .there can b 1 or more chiral centres in a molecule. the molecule will show opticl activity depending on presence of axis/centre/plane of symmetry.it can b carbon or N or any other atom .whrereas stereocentre may b that atom of a mol vich show optical activity.           u can refer 2 morrison boyd for details but i sujest use it only for refernce