Raga or Raag is a framework created out of surs or swaras. Each raga is said to affect an emotion in the minds of listeners. Each raga has a unique set of sequence of notations (sur) which characterizes that particular raga.
Raga is a loose framework as such in the sense that it can be sung in as many variations as possible as long as the basic rules of layout of surs in that raga are followed.
A singer can thus embellish a raga in a variety of ways by adding other notations (sur) while maintaining the core notational framework of raga.
This also means that a Raga can take various forms and it is really upto singer to use his imagination and experience in embellishing the raga with as many variations in surs as possible.
Thus there are many ways in which a raga can be described. Each way describes the raga in a different level of details.
Some ways explain in lesser detail and some in larger detail. You can think of this as an information pyramid of ragas.
The raga descriptors at the top of pyramid gives minimal information while those towards the bottom give more information
As you can see in the picture above, there are five levels of details of a Raga – Thaat, Aaroh/Avroh, Pakad, Chalan and Vistaar.