Please give me a short description of each of the three categories of performance that Bharatanatyam follows.
1. Nritta: This is the pure movement aspect of the dance of Bharatanatyam. The dancer showcases movements in varying form, speed, range and levels. 
2. Nritya: This is the expressive aspect of the dance, wherein the dancer tells a story, usually of a spiritual theme of Hindu tradition, through both facial expressions and movements.
3. Natyam: This is the theatrical aspect of the dance, where the dancer incorporates the elements of Nritya, to enact different characters of a play. This can be performed either solo or as a team.