A thaat is a classification of Raags. Its a superset over the set of raags, kind of like a family of raags.
Every raag belongs to a particular thaat, for example, Raag Yaman belongs to Kalyan thaat, Raag Puriya Dhanashree belongs to the Poorvi thaat. What this means is that the raag within a thaat uses a part of all the notes that are defined in the thaat. For example, the Kalyan thaat uses the notes Sa Re Ga Ma* Pa Dha Ni. The Raag Yaman uses the same notes, however it has some added rules on top of it. Sarang is another raag belonging to the Kalyan thaat. It however only uses the notes Sa Re Ma* Pa Dha Ni. Notice that it does not use the Ga.
Most of the thaats are raags themselves. For example Khamaaj is a thaat and a raag.
A thaat defines the scale or the notes allowed within that family. A raag adds its own rules and boundaries utilizing the notes within that scale.
There are some raags which belong to more than one thaat. For example the Yamani-bilawal raag belongs to both Kalyan and Bilawal thaat.
Below is the scale of all the 10 thaats in Hindustani Classical Music. Click on them and get a feel of how different they sound just moving up and down the octave.
Thaat helps us to place a raag and to figure out its structure in terms of the notes it is bound with. When you hover over a particular thaat, and listen carefully to the ascent and descent, you will surely feel the shift in the mood and the note structure. All the ascent and descent is just Sa Re Ga Ma Pa Dha Ni Sa Sa Ni Dha Pa Ma Ga Re Sa, but the different set of combinations that arise out of the komal and shuddha notes makes it a vast musical treasure to explore.
There are a lot of bugs in this page, I will clear them out as and when I get the knowledge and time to do so :) Until then, do come back with comments and suggestions about what I can do more with this page. I wish to add a list of songs which belong to a particular thaat, probably by making the chart more interactive. Lets see how far I can go with this.