Shayera Hol (and all her various spellings) is HawkgirlandHawkwoman. She started out as Hawkgirl, then went to Hawkwoman, but I believe went back to Hawkgirl. Hawkgirl is usually the mantel she wears though.
Shayera Thal, is alsoHawkwoman. Albeit, not as long as Shayera Hol. Shayera Thal's storyline I don't quite understand completely. It's confusing to me personally so I suggest looking it up on wikipedia or something. From my understanding she was also with Katar Hol and was the daughter of Shayera Thal I. Currently, Shayera Thal is a supervillain after DC reimagined the line in 2011.
Sharon Parker also carried the name Hawkwoman. She was an ordinary earth girl who met a Thanagarian spy and was mindwiped and then forced to pretend to be Carter Hall's (Hawkman) wife. Sharon is also the mother of Golden Eagle.
Now Kendra Saunders is also Hawkgirl. Kendra had commited suicide and when her soul left, Shayera Hol's replaced it. However, she didn't have Shayera's memories, but did have Kendra's, so, she believes herself to be Kendra. Her storyline is also another I recommend going to wikipedia for.
Knowing all this you must also realize that with every comic writer you run the risk of getting yet another incarnation of the same character. Many times Shayera Hol has been different from Shayera Hall and sometimes she's even the same person as Shayera Thal. Same with Hawkman, he has been Katar Hol and Carter Hall seperately, but other times they are the same person. And, at one point in time, Hawkman was Shiera Hall, Carter Hall, and Katar Hol merged together.
It's fairly confusing.
Personally for me, and many other comic fans in general, Shayera Hol is the one and only Hawkgirl. And Hawkwoman was just never a big deal.