An official recognition that a person is allowed to vote. Voter registration cards generally tell the holder his or her voting precinct and the elections for which he or she can cast a vote--the cards might state which City Council positions and which Congress races the person will cast a ballot in. Registration cards, theoretically at least, are an accurate reflection of a person's place of residence.
US registration cards can generally be received by filling out a registration form from the County Board of Elections.