A circle of latitude is an imaginary circle around the Earth made up of all the points that have the same particular value for their latitude. These are based on the rotation of the Earth in relationship to the Sun.
The five major circles of latitude are:
great circle. The other lines of lattitude are not the shortest distance between points (it is for this reason that an aeroplane travelling between a European and North American city on the same lattitude will fly further north, over Greenland for example), and are thus not really "lines" in the geometry of the sphere.
A circle of latitude is often referred to as a "parallel".