What is the difference between jinn & seytan in Islam?"
The jinn are among the creatures that Allah (swt) created. The jinn race was created from fire.
"And the Jinn race, We had created before, from the fire of a scorching wind." (Quran 15:27)
They have intellect, freedom and power to choose between right and wrong. These characteristics suit the need for them to worship God.
"I have only created Jinns and men, that they may serve Me." (Quran 51:56)
They are being hold responsible for all their actions like humans do in the Judgement Day.
"On that Day no question will be asked of man or Jinn as to his sin." (Quran 55:39)
The jinn with sins are going to be placed in Hell.
He will say: "Enter ye in the company of the peoples who passed away before you - men and jinns, - into the Fire." (Quran 7:38)
Satan on the other hand is the enemy to mankind.
"Said (the father): "My (dear) little son! relate not thy vision to thy brothers, lest they concoct a plot against thee: for Satan is to man an avowed enemy!" (Quran 12:5)
Satan are consisted of both men and jinns.
"Likewise did We make for every Messenger an enemy,- evil ones among men and jinns, inspiring each other with flowery discourses by way of deception. If thy Lord had so planned, they would not have done it: so leave them and their inventions alone." (Quran 6:112)
Yes, both can posses people.
"Those who devour usury will not stand except as stand one whom the Evil one by his touch Hath driven to madness..."
Iblis is a jinn and satan both at the same time.
"But Satan whispered evil to him: he said, "O Adam! shall I lead thee to the Tree of Eternity and to a kingdom that never decays?"
""Bow down to Adam": They bowed down except Iblis. He was one of the Jinns..."
Allah knows best
May Allah guide us all