1. Shaytan can lead people astray, esp. those who have weak faith.
2. Whispering into his heart and telling him to do forbidden things.
3. Shaytan considered himself to be better than Adam (AS) because shaytan was made of smokeless fire, while Adam (AS) was made of clay.
4. Nightmares and disliked dreams are from Shaytan. However, we shouldn't really discuss them with anyone, or act upon it if it goes against Islam.