The Iraqi insurgency is composed of at least a dozen major guerilla organizations and perhaps as many as 40 distinct groups. These groups are subdivided into countless smaller cells. Due to its clandestine nature, the exact composition of the Iraqi insurgency is difficult to determine. Because they are civilians against an organized State army many considered them a guerrilla fight. It is often divided by analysts into several main ideological strands, some of which are believed to overlap:
Ba'athists, the armed supporters of Saddam Hussein's former nomenclatura, e.g. army or intelligence officers;
Nationalists, mostly Sunni Muslims, who fight for Iraqi self-determination;
anti-Shi'a Sunni Muslims who fight to regain the prestige they held under the previous regime (the three preceding categories are often undistinguishable in practice);
Sunni Islamists, the indigenous armed followers of the Salafi movement, as well as any remnants of the Kurdish Ansar al-Islam;
Foreign Islamist volunteers including the often linked to al Qaeda and largely driven by the Sunni Wahabi doctrine (the two preceding categories are often lumped as "Jihadists");
Patriotic Communists (who have split from the official Iraqi Communist Party) and other leftists;
Militant followers of Shi'a Islamist cleric Moqtada al-Sadr, very active members in the April 2004 campaign, have later begun to take over a more passive role, providing logistical and moral support, but this may change again;
Criminal insurgents who are fighting simply for money; and
Nonviolent resistance groups and political parties (not technically part of the insurgency).
Ba'athists
The Ba'athists include former Ba'ath Party officials, the Fedayeen Saddam, and some former agents of the Iraqi intelligence elements and security services, such as the Mukhabarat and the Special Security Organization. Their goal, at least before the capture of Saddam Hussein, was the restoration of the former Ba'athist regime to power. The pre-war organization of the Ba'ath Party and its militias as a cellular structure aided the continued pro-Saddam insurgency after the fall of Baghdad, and Iraqi intelligence operatives may have developed a plan for guerrilla war following the toppling of Saddam Hussein from power. Following Saddam's capture, the rhetoric of the Ba'athist insurgents gradually shifted to become either nationalist or Islamist, with the goal of restoring the Ba'ath Party to power as it once was seemingly out of reach. Many former Ba'athists have adopted an Islamist façade in order to attract more credibility within the country, and perhaps support from outside Iraq. Others, especially following the January 2005 elections, became more interested in politics.
Many Ba'athist organizations, such as the Fedayeen Saddam, have a violent past. Saddam used this particular group as a way to silence his political opponents into submission through fear. One such terror campaign involved members of the Fedayeen Saddam systematically beheading female family members of opponents of his regime.
Nationalists
Nationalists from the Sunni Arab regions are drawn from former members of the Iraqi military as well as other Sunnis. Their reasons for opposing the coalition vary between a rejection of the foreign presence as a matter of principle to the failure of the multinational forces to fully restore public services and to quickly restore complete sovereignty. Some Iraqis who have had relatives killed by coalition soldiers may also be involved in the insurgency. Most likely, the majority of the low-level members of the indigenous Sunni insurgency (such as foot soldiers) fall under this broad category. A smaller number of Shi'a nationalist fighters also exist, which are usually recruited from left-wing backgrounds. Sunni nationalists are mainly left-wing or, more commonly, ex-regime adherents.
Some of these insurgents pursue the restoration of the power previously held by the Sunni minority in Iraq, who controlled all previous Iraqi regimes since the departure of the British. One former minister in the interim government, Ayham al-Samarai, "launched a new political movement, saying he aimed to give a voice to figures from the legitimate Iraqi resistance. 'The birth of this political bloc is to silence the skeptics who say there is no legitimate Iraqi resistance and that they cannot reveal their political face,' he told a news conference." <1> There are some groups of Sunni Islamists who have taken a more explicitly anti-Shi'a role and frequently engage in revenge killings; these are mainly vigilante groups of local significance (as most of their Shi'a counterparts).
http://en.wikipedia.org/wiki/Iraqi_Resistance