Israeli forces killed a leader of a Salafi faction in the Gaza Strip on Friday, the second such strike on Palestinian militants with al Qaeda ties this week. Here are some facts about the constellation of ultra-conservative Islamists known as Salafis, who have challenged Hamas' rule in the isolated enclave. Groups -- Jaysh al-Islam (the Army of Islam) is closely linked to Gaza's powerful Doghmush clan, which worked with Hamas to capture Israeli soldier Gilad Shalit in 2006 but broke with it over the Doghmushes' four-month kidnap of a BBC journalist in 2007. -- Tawheed and Jihad (One God and Holy War), linked by Gazan sources to the obscure "Jihadist Salafi" group, that in April abducted and killed a pro-Palestinian activist from Italy, Vittorio Arrigoni. Tawheed and Jihad also praised an August attack by infiltrators that killed eight Israelis on the Egyptian border. Israel blamed that raid on the Palestinian militant faction Popular Resistance Committees, killing five of its members in retaliation. The PRC denied involvement. -- Ansar al-Sunna (Followers of al-Sunna, the words and deeds of the Prophet Mohammad), which carried out a lethal rocket attack on Israel last year and whose name had been used by, among others, al Qaeda-allied Sunni insurgents in Iraq. -- Jund Ansar Allah (Warriors of God), which raided an Israeli border post on horseback. -- Jaysh al-Ummah (Army of the Muslim Nation), whose leader, Abu Hafs, was detained by Hamas. -- Jaljalat (Rallying Cry), which includes former Hamas members and is suspected of bombing several Internet cafes as part of its especially hard line against Palestinian "apostates". How do Salafis differ from Hamas Islamists? -- The Salafis, whose diffuse networks may be designed to evade Hamas crackdowns, share the goal of fighting Western powers and founding a purist Islamic state across the Middle East. Though Hamas echoes al Qaeda's calls to destroy Israel, its ambitions are framed within Palestinian nationalism and include political accommodation with secular rivals and a possible truce with the militarily superior Israel. Hamas has refrained from imposing sweeping Islamic law since taking over Gaza in 2007 and has condemned al Qaeda attacks abroad. -- Membership of the Salafi groups appears to number in the hundreds but with potentially thousands of supporters among Gaza's 1.5 million population. They have been reinforced by volunteers who slip in through the neighboring Egyptian Sinai, where security has eroded amid political upheaval in Cairo. -- Hamas, which won a parliamentary election in 2006, has some 25,000 men under arms in Gaza. There has been disillusion with its rule and an Israeli-led embargo which has ravaged the economy. Hamas has been publicly tolerant of Salafis, saying they are misguided and offering them "re-education". But in practice it has often tried to rein in Salafists intent on provoking Israel with cross-border attacks or violently undermining Hamas authority. In the bloodiest confrontation, Hamas forces stormed a mosque in the southern border town of Rafah in August 2009 after a Jund Ansar Allah preacher and leader, Abdel-Latif Moussa, publicly declared Gaza to be an Islamic emirate. -- Among other Salafi complaints about Hamas are its tolerance of Gaza's 3,000-strong Christian community and the backing Hamas receives from Shi'ite Iran. Most Palestinians, including Hamas members, are Sunnis. Salafis are also blamed for attacks on people and groups they see as defying religion, including Internet cafes.