The Krithi or composition - lyric set to a raagam and thaalam is the basis of Carnatic Music. The Krithi is said to be an evolution of the Keerthanai or devotional songs which were simple in melodic structure, had a number of stanzas and emphasized devotion (Ex. Thyaagaraaja's Uthsavasampradaaya Keerthanas). The Krithi is more sophisticated and as evolved now has the Pallavi (refrain), Anupallavi (the second part developing on the theme set by the Pallavi) and Charanam (a larger part, usually starting at lower region of the octave). They have many 'Sangathi's (repetitions with progressive variations in melody). Now often the terms Krthi and Keerthana are used interchangeably. The Krithi form can be sent to have blossomed into full glory in the musical trinity's compositions. In one form of krithi (most of Thyaagaraaja Krithis) the second half of the Charanam has the same melodic structure as the anupallavi.
Some Krithis have more than one Charanam with identical melody (and occasionally with different melodic structures as in Paraasakthi Janani in Hamsadhwani by Paapanaasam Sivan). Some Krithis of Dheekshithar do not have Anupallavi but only a 'Samashti Charanam' (ex. Mahaaganapathim in Naata). Dheekshithar's compositions, though well structured, in general do not follow a set melodic pattern; the Charanams are large with unexpected melodic turns.
The Krithis along with Varnams have largely contributed to defining the Raaga Lakshanam, as mere Aarohanam and Avarohanam cannot fully describe the finer nuances of a Raagam.
In Carnatic Music the Raagam mood can be conveyed with a few characteristic phrases and this has enabled composing 'Raagamaalika' - Krithi with a succession of raagams - each Raagam for a few Aavarthams or even for one Aavartham. In many Raagamaalikaas, Chitta Swarams are added for each Raagam and at the end Chittaswarams are sung in the reverse order ('Viloma kramam') ending with the Raagam of pallavi (as in 'Bhaavayami Raghuraamam'). In many raagamaalikaas, the name of the Raagam (raaga mudhra) is found in each of the stanzas. Among the trinity Dheekshithar has composed raagamaalikaas (ex. Sri Viswanaatham in 14 raagams with Raagam changing in the middle of aavartham). Navaraagamaalika Varnam (in 9 raagams) is a standard varnam taught to students. Maha Vaidhyanaatha Iyer composed raaga thaala maalikaa in the 72 melakarthaa raagams and with variation in thaalam also. In recent times Swaathi Thirunaal's 'Bhaavayaami' is sung as raagamaalikaa. 'Sri Chakraraaja' is an example of Raagamaalika without chittaswarams.
Apart from compositions set as Raagamalikaas by the composer himself, it is also customary to sing Sanskrit Slokams and Thamizh 'Viruthams' (verses not set to any Thaalam) as Raagamaalikaas at the end of a concert. Some of Subramanya Bharathiar's songs have also be set as Raagamaalikaas.