Latitude and Longitude are the coordinates we use to find where things are on Earth.  

     Right Ascension and Declination (RA/Dec) are the coordinates we use to find where things are in the sky.  

     Declination is similar to Latitude.  Latitude measures how far you are from the equator in degrees.  The north pole is at +90 degrees and the south pole is at -90 degrees.  Declination uses the celestial sphere.  The North Star, Polaris, is Declination's "north pole" at +90 degrees.  Declination's "south pole" is at -90 degrees.  

     Right Ascension is similar to Longitude.  Where Longitude uses the Prime Meridian (the original site of the Greenwich Observatory in Greenwich, England), Right Ascension uses the Sun's location at the Vernal Equinox.  The Vernal Equinox is where the Sun crosses the celestial equator on March 21.  Right Ascension is measured from this point, all the way around the sky until the Vernal Equinox is reached again.  Longitude is measured from -180 to +180.  Right Ascension never uses a minus, and is measured in units of time.  The entire distance around the sky is one day.  Measurements are made in hours, minutes and seconds.