First of all, it is to be stated that Islam forbids abortion in all stages of pregnancy. When the pregnancy reaches 120 days, abortion becomes totally forbidden and is deemed a form of murder that results in compensation becoming liable. If abortion is done prior to this period, no compensation is liable but one should ask forgiveness and promise never to commit it again. In addition, it is highly recommended for such an individual to do much righteous deeds such as giving in charity, etc.
However, the only condition under which abortion is allowed is when there is an actual threat to the life of the mother confirmed by an official medical report that if the pregnancy advances any further, the mother may die.

The Qur'an clearly disapproves of killing other humans: “Take not life which Allah has made sacred” (6:151; see also 4:29 “If a man kills a believer intentionally, his recompense is Hell, to abide therein (for ever)” (4:93). Allah (SWT) went even further, making unlawful killing of a single individual human being equal to mass murder of the whole of mankind: "Because of that, We ordained for the children of Israel that if anyone killed a person not in retaliation for murder or for spreading mischief on earth, it would be as if he killed all mankind. And who saved a life, it would be as if he saved all mankind." (Al-Maidah, 5:32)

As to whether abortion is a form of killing a human, the Qur'an does not make any explicit statements. Only Chapter/Surah 17:31 warns believers in general: “Kill not your children for fear of want. We shall provide sustenance for them as well as for you. Verily the killing of them is a great sin.”