From a long letter on the jubilee issued today:
The tragedy of abortion is experienced by some with a superficial awareness, as if not realizing the extreme harm that such an act entails. Many others, on the other hand, although experiencing this moment as a defeat, believe that they have no other option. I think in particular of all the women who have resorted to abortion. I am well aware of the pressure that has led them to this decision. I know that it is an existential and moral ordeal. I have met so many women who bear in their heart the scar of this agonizing and painful decision. What has happened is profoundly unjust; yet only understanding the truth of it can enable one not to lose hope. The forgiveness of God cannot be denied to one who has repented, especially when that person approaches the Sacrament of Confession with a sincere heart in order to obtain reconciliation with the Father. For this reason too, I have decided, notwithstanding anything to the contrary, to concede to all priests for the Jubilee Year the discretion to absolve of the sin of abortion those who have procured it and who, with contrite heart, seek forgiveness for it. May priests fulfill this great task by expressing words of genuine welcome combined with a reflection that explains the gravity of the sin committed, besides indicating a path of authentic conversion by which to obtain the true and generous forgiveness of the Father who renews all with his presence.
He also announces this significant news regarding SSPX:
A final consideration concerns those faithful who for various reasons choose to attend churches officiated by priests of the Fraternity of St Pius X. This Jubilee Year of Mercy excludes no one. From various quarters, several Brother Bishops have told me of their good faith and sacramental practice, combined however with an uneasy situation from the pastoral standpoint. I trust that in the near future solutions may be found to recover full communion with the priests and superiors of the Fraternity. In the meantime, motivated by the need to respond to the good of these faithful, through my own disposition, I establish that those who during the Holy Year of Mercy approach these priests of the Fraternity of St Pius X to celebrate the Sacrament of Reconciliation shall validly and licitly receive the absolution of their sins.
Until the doctrinal questions are clarified, the Society has no canonical status in the Church, and its ministers – even though they have been freed of the ecclesiastical penalty – do not legitimately exercise any ministry in the Church.
UPDATE: A priest friend noted that many bishops in the United States and Canada have already given their priests permission to forgive the sin of abortion. The pope’s letter makes this practice, for the Year of Mercy, universal. The National Catholic Register notes:
On behalf of the Vatican, Fr. Thomas Rosica CSB later issued the following note regarding Pope Francis’ words on forgiveness of the sin of abortion following many enquiries from reporters:
“Forgiveness of the sin of abortion does not condone abortion nor minimize its grave effects. The newness is clearly Pope Francis’ pastoral approach. Many bishops have granted priests permission to forgive the sin. The fact that this statement is coming from the Pope and in such a moving, pastoral way, is more evidence of the great pastoral approach and concern of Pope Francis. That people come to confession today to confess abortion and other grave sins is cause for us in the Church to thank God and to put into practice the mission of the good and merciful shepherd who came to seek out those who were lost.”
Q. 1. Does the Catechism of the Catholic Church teach us anything about how a Catholic can be forgiven of the sin of abortion?
A. 1. The following two sections of the Catechism of the Catholic Church state:
# 2272 Formal cooperation in an abortion constitutes a grave offense. The Church attaches the canonical penalty of excommunication to this crime against human life. “A person who procures a completed abortion incurs excommunication latae sententiae,” [CIC, can. 1398] “by the very commission of the offense,” [CIC, can. 1314] and subject to the conditions provided by Canon Law. [Cf. CIC, cann. 1323-1324] The Church does not thereby intend to restrict the scope of mercy. Rather, she makes clear the gravity of the crime committed, the irreparable harm done to the innocent who is put to death, as well as to the parents and the whole of society. 
# 1463 Certain particularly grave sins incur excommunication, the most severe ecclesiastical penalty, which impedes the reception of the sacraments and the exercise of certain ecclesiastical acts, and for which absolution consequently cannot be granted, according to canon law, except by the Pope, the bishop of the place or priests authorized by them. [Cf. CIC, cann. 1331; 1354-1357; CCEO, can. 1431; 1434; 1420] In danger of death any priest, even if deprived of faculties for hearing confessions, can absolve from every sin and excommunication. [Cf. CIC, can. 976; CCEO, can. 725] 
Q. 2. What does that mean in simple English?
A. 2. It means that anyone who has an abortion, participates in an abortion, or supports an abortion, he or she, by the previously mentioned actions, is automatically excommunicated from the Catholic Church. (C.C.C. # 2273)
Accordingly, that person can no longer receive any of the Sacraments of the Catholic Church, be it the Sacrament of the Holy Eucharist (Communion), Marriage or the Sacrament of Confession. For an excommunication to be lifted (ended), the sinner must approach the local Bishop, inform him of his/her “latae sententiae” excommunication through the sin of abortion and ask the Bishop to have the excommunication lifted (ended) and to be absolved of the sin of abortion. The Bishop may authorize a priest to specifically deal with this absolution on his behalf. (C.C.C. # 1463) Contrary to the popular belief of Catholics, this does not mean that all priests can absolve a penitent of the excommunication. Only a priest who is specifically appointed by the Bishop to do so, may do so. (Emphasis added.)
Q. 3. Does that mean that if a woman had an abortion and confessed it to a priest who had not been specifically appointed by the local Bishop to lift her excommunication, that her sin was never forgiven?
A. 3. That is correct! There have been many instances where priests did not realize that an abortion incurs an automatic excommunication. As such, while they may enjoy the authority to forgive sins, they do not have the authority to lift excommunications. Consequently, while a person is excommunicated, that person cannot validly receive any of the Sacraments, that including the Sacrament of Confession. (Exception: In danger of death any priest, even if deprived of faculties for hearing confessions, any priest can absolve from every sin and excommunication.)
UPDATE II: John Allen has his own thoughts on the “daring double play” of the “Pope of Mercy.” Check it out.