I heard that latest style is AAA. Is there any other one? Why we use one and don't use another?
I think there are 2 major styles - one is the classic one (not sure what is the name) - which existed from the beginning and the earliest versions of .Net (certainly I used it with .Net 1.1) and the AAA one. Both are still fully functional, so feel free to use any (but you can't mix them - it simply doesn't work and you will get weird errors).
The AAA style was developed after lambda expressions and extension methods were introduced in C# and many people (including me) think it is cleaner, easier to read and expresses the intent of the test better.