Nitya karma refers to those karmas (or rituals) which have to be performed daily by Hindus. The Hindu Shastras say that not performing nitya karmas leads to sin. The nitya karmas include:

• Snana (bathing
  Abhisheka
  Abhisheka is a Sanskrit term comparable to puja, yagya and arati that denotes: a devotional activity; an enacted prayer, rite of passage and/or religious rite or ritual...
  
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• Sandhyavandanam
  Sandhyavandanam
  Sandhyavandanam is a religious practice performed by Hindu men initiated into the rite by the ceremony of Upanayanam, and instructed in its execution by a Guru . Sandhyavandanam consists of excerpts from the Great Vedas that are to be recited thrice daily...
  
  
• Devataarchanam
• Aupasana
  Aupasana
  Aupasana is a yajna performed on a daily basis by Hindus who have been initiated during marriage for this.Hindu marriage is conducted with offerings made in the fire. Aupasana, which must be performed every day, is commenced in this fire and it must be preserved throughout one's life...
  
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• Agnihotra
  Agnihotra
  Agnihotra is a Vedic yajña performed in orthodox Hindu communities. It is mentioned in the Atharvaveda and described in detail in the Yajurveda Samhita and the Shatapatha Brahmana . The Vedic form of the ritual is still performed Nambudiri Brahmins of Kerala and by a small number of Vaidiki...
  
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Nitya Karma does not necessarily mean daily duties. It includes any regular/periodic scheduled activities/duties. E.g.: Amavasya tharpanam, Grahana tharpanam, Pithru devasam.

There is a subdivision of Nithya karma which is called Nai-Nithya karma. This means compulsory karma but conditional. E.g. Grahana related karmas