What is the difference between tanwin and nunussakina?

Question
What is the difference between tanwin and nunussakina?

Answer
They are both saakinah, but their appearance is different and placement is different.  For example, the tanween is not a written , but symbolizes a and is only found on the end of nouns.  It is not part of the basic make up of the word.
The noon saakinah can occur in verbs, nouns, prepositions and particles, and can be in the mididle or the end of a word.
For more information, please see the following link
http://www.abouttajweed.com/the-noon-saakinah-and-tanween/noon-saakinah-and-tanween.html