What are the harsh and soft letters in the Arabic alphabet?

Question
What are the harsh and soft letters in the Arabic alphabet?

Answer
There are different characteristics for the letters in the Arabic alphabet.  We assume you are referring to  , which means strong, and , which means softness, and a third characteristic between these two, which is , meaning moderate.
You can read about these characteristics and their letters in the following link:
http://www.abouttajweed.com/the-characteristics-of-the-arabic-letters/the-characteristics-of-the-letters-part-3-ash-shiddah-at-tawassit-ar-rakh.html