Vivadi and Samvadi Swaras

Vadi Swara:
Vadi Swara is an important or predominant note in a raaga. It is also called the life note. Different notes determined as Vadi notes can express different emotions at various times of the day and night. A Vadi note is often addressed as the king and will be repeated several times and emphasized during the rendering of the raaga.

Samvadi Swara:
Subsonant note. This is the second most important note of a Raga and In most cases the Vadi and Samvadi notes are a fourth or fifth apart.
For example, Shadja and Suddha Madhyama, Suddha Rishabha and Suddha Dhaivata, Suddha Gandhara and Suddha Nishadha, Sadaharana Gandhara and Kaisiki Nishada, Antara Gandhara and Kakali Nishada, Suddha Rishaba and Prati Madhyama are samvadi swaras.

Anuvadi Swara:
Assonant note. The notes in a Raga that are neither Vadi or Samvadi are called its Anuvadi notes. They are often addressed as companion or attendant notes.

Vivadi Swara:
Dissonant note. It is a note which should not be used in a Raga. As an exception however, it is used by skillful musicians in an implied form in order to enhance the beauty of the Raga.
Vivadi notes are often addressed as enemy notes. Chatshruthi Rishabam,Sudha gandaram, chatsruthi deivatham, sudha nishadam are considered as vivadi swaras and the raagas which has any of these are vivadi raagas.

Here is a video where Nityasree Mahadevan explains the vivadi and Samvadi Swaras in a raaga.