What's a 'sanctuary city,' and why should you care?

The killing last week of a San Francisco woman, allegedly by an undocumented immigrant with a felony record, has put the spotlight on the city's policy of refusing to honor federal requests to hold on to people found to be in the country illegally.

San Francisco authorities released suspect Juan Francisco Lopez-Sanchez in April after dropping the drug charges on which they had asked federal authorities to turn him over -- even though federal officials had asked the city to let them know if they were going to cut him...