Meg was possessed by Azazel's "daughter" while she was in college.  We met first met her when she was hitching to California in Episode "Scarecrow".  She was later exorcized but crawled out to possess Sam in "Born under a Bad Sign".  In which she was exorcized once again back to hell.  

Meg appears again as herself, the young girl before she was possessed, in "Are you there God? It's me, Dean Winchester" as one of the witnesses who visit the brothers after the seal is broken.  It is then we find out that Meg Masters was "awake" for all the evil doing that the demon possessing her did.  We also find out that her little sister, who worshiped her, killed herself after Meg's disappearance.

Tammi is one of the more powerful black eyed demons.  As seen in "Malleus Maleficarum" Tammi is able to pin and hold both Dean and Sam to the wall at the same time.  She is also able to kill from a distance and is usually the demon that people sell their souls to when they become Witches. 

Tammi is also the demon who took Ruby's soul when Ruby was a witch.  She is killed by Dean using Ruby's knife.