Latitude, represented by a series of horizontal lines on a map - or horizontal rings on a globe - is the angular distance of a point on the Earth either north or south of the Equator. There are 180 degrees of latitude, 90 north and 90 south of the equator, which is defined as 0°. Lines of latitude are often refered to as "parallels". N90° and S90° coincide with the north and south poles, respectively.
Why don't lines of Lat/Lon form a square angle on a map? Because the meridians converge at the north and south poles. If you divide the earths' circumference at the Equator and through any meridian by 360, the distance on the earths' surface for each degree of longitude and longitude at the equator is approx 69 miles or 111 kilometres. As you move either north or south of the equator the meridians get closer together until they converge at the poles. At N45° & S45° one degree of longitude is approximately 49 miles or 78 kilometres. One degree of latitude is still 69 miles. Another way to explain it is that all the meridians are of a constant circumference whereas the parallels reduce in circumference as you move north or south.
See also coordinates.