Wikipedia
Shafa'ah (, "intercession"; ; ), in Islam is the act of pleading to God by an intimate friend of God (Muslim saint) for forgiveness. Shafa'ah has a close meaning to Tawassul, which is the act of resorting to intimate friends of God to ask forgiveness.
The word Shafa'ah is taken from ''shaf '' which means even as opposed to odd. The interceder, therefore, adds his own recommendation to that of petitioner so that the number of pleaders becomes even. Meaning the weak plea of the petitioner become strong by the prestige of the intercessor. Accordingly, Shafa'ah is a form of prayer to request God by the sake of those who are near to Him in order that as a member of the believing community one could hope for the intercession of the intercessors and hence deliverance from eternal damnation though not necessarily from temporary one.
Shafa'ah has lately come to be among the most controversial concepts within Islamic thought. This is because some verses of the Quran negate it, if taken into consideration alone, stating that no intercession would be accepted in the day of resurrection. However, some other verses confirm it declaring that only God has the right to intercede in the next life. Finally, a third kind of verses state that some people have the authority to intercede by permission of God. Wahhabies, taking the first two kinds of these verses as true, believe that there is no intercessor but Allah, and say that whoever believes in intercession of anyone other than God is not a Muslim, rather is a polytheist ( heretic). Others believe that while intercession should not be a means of emboldening people to committing sins, it should be considered as a ray of hope which lead sinners to the right path after they have wronged themselves. Belief in the intercession of Muhammad developed after his death by both Sunnis and Shiites, however among the Shiite the idea of mediation was extended to include the The Twelve Imams and other intimate friends of God too.