The apparent path of the Sun as seen from the Earth, the ecliptic is a great circle of 360 degrees longitude, which is divided up into 12 signs of 30 degrees each. All planetary motion is measured against this circle. The start of the circle is 0 degrees Aries, the position of the Sun at the Vernal Equinox which occurs around 21 March each year.
The planets are also measured in latitude [declination] not shown.