Each one of us is born under one of 27 nakshatras. This is also known as one’s janma nakshatra or birth nakshatra.
Some also refer to this as the birth star even though that terminology is not quite precise since a nakshtra can be much more than just one star.
27 Nakshatras in a Great Circle
Imagine a great circle in space and the 27 nakshatras laid out equally on this circle. Since a circle covers 360 degrees, these 27 nakshatrams cover 360 degrees in space. So the “space” occupied by one nakshatram is 360 divided by 27 which is equal to 13 degrees and 20 minutes.
12 Raasis = 27 Nakshatras!
1 Raasi = 2 and 1/4 Nakshatras
The space occupied by the 12 rashis is the same space occupied by the 27 nakshatras. Thus each one of us has a janma rasi just like we have a janma nakshatra.
Since 12 rasis = 27 nakshatras, one raasi = two and a quarter nakshatras (since 27 divided by 12 = 2 and 1/4). In other words, the 27 nakshatras are grouped under 12 rasis at two and a quarter nakshtras per raasi.
As you can see, a nakshatra can occupy a part of one rasi and a part of the next rasi (as laid out in the table below). This is the case for 9 of the 27 nakshatras. These 9 are: Kritika, Mrigasira, Punarvasu, Uttara Phalguni, Chitra, Visaka, Uttarashada, Dhanishta and Poorvabhadra are partly in one rasi and partly in another.
But if each nakshatra is divided into 4 equal quarters, a quarter will never occupy more than one rasi. This higher precision division of each nakshatra into quarters has been done.
Nakshatra Paadham = 4 Quarters of each nakshatra
Since there are 27 nakshatras, there are 108 nakshatra quarters (27 x 4 = 108) in total with each quarter spanning 3 degrees and 20 minutes of the great circle.
Each quarter of each nakshatra will never span two rasis; each quarter will be part of one and only one rasi. Each such nakshatra quarter is referred to as a Nakshatra Paadham or Nakshatra Pada.
When you hear people make statements such as “My nakshatra is Punarvasu 2nd padham” or “My birth star is Vishaka 4th quarter” and so on, they are referring to this finer division of each nakshatra into quarters.