Some atheists  distinguish between two variants:
(1) Weak atheism aka negative atheism is the standpoint that there is no reason to believe that God exists/there are any gods. (One may hold that there is not enough evidence to support a decision, or one may simply not have a position). This is related but not equivalent to agnosticism, which affirms that a person cannot have firm knowledge of the existence or the inexistence of any deity. Nonetheless, the two terms are often used interchangeably.
(2) Strong atheism aka positive atheism is the standpoint that there is reason to believe that God doesn't exist/there are no gods. This may include the view that the existence of God or gods is even logically impossible. Such a position usually commits one to having, or at least allowing for, positive assertions about and explanations of the natural world which do not require a deity.
"Weak atheism" as defined above is also referred to in some circles as "weak agnosticism" while "agnosticism" as defined above is specified as "strong agnosticism." Moreover, in the freethought tradition, strong and weak atheism are also called positive and negative atheism.
Strong atheism extends the viewpoints of weak atheism, but weak atheism doesn't entail the strong form. The strongest form of positive atheism, which holds that it is impossible that God exists/there are any gods, is based on logical a priori arguments that indicate the monotheistic conception of God is self-contradictory or internally inconsistent, and, hence, cannot describe anything real.
Atheism is not necessarily synonymous with irreligion. There are religious belief systems, including much of Buddhism, Unitarian Universalism, and Universism, which do not require belief in a deity, and Jainism, which requires strong atheism. Additionally, a number of atheistic "churches" have sprung up, as have religious organizations which allow atheists as members. The "pan-atheists" who call themselves Naturalistic Pantheists are one example, while Brianism is another.
For most religious people, religion is an important part of their morality; therefore, it is possible to think that without religion, there is no morality. Most atheists vehemently deny the charge that they are amoral and point to the moral code of Secular humanism. Others see this as an inaccurate assessment, pointing out the number of charitable religious organizations such as the Salvation Army. Atheists and other secularists would point to useful secular charities like Oxfam and Médicins Sans Frontières (Doctors Without Borders). Since atheists very rarely contribute to charities "in the name of atheism" but may contribute just as much as theists for other reasons, it may be inaccurate to measure overall charitableness in this manner.
Many atheists and indeed some theists consider that morality does not require a religion. Francis Bacon explains: "Atheism leaves a man to sense, to philosophy, to natural piety, to laws, to reputation; all of which may be guides to an outward moral virtue, even if religion vanished; but religious superstition dismounts all these and erects an absolute monarchy in the minds of men." .
In some cultures, promoting atheism has been criminalized, and even many western European countries such as Germany and Spain still have (rarely enforced) anti-blasphemy laws on the books. Those who hold theistic views often consider those without a belief in a deity to be immoral, amoral or untrustworthy—unfit as members of society. The scriptures of most religions contain denunciations of non-believers; see, for example, 2 Thessalonians 1:6-9 in the Christian Bible.
Every military buildup in the United States since World War II has been accompanied by frequent use of the saying "There are no atheists in foxholes." During the Cold War, the fact that the communist enemies of the United States were officially atheists ("Godless Communists") added to the view that atheists were unreliable and unpatriotic. As recently as the 1988 presidential campaign in the (officially secular) United States, George H. W. Bush said "I don't know that atheists should be regarded as citizens, nor should they be regarded as patriotic. This is one nation under God."  Similar statements were made during the controversy surrounding the inclusion of the phrase "under God" in the American Pledge of Allegiance, words which were added to the pledge early in the Cold War period.
In the Soviet Union and in the People's Republic of China, some churches that submitted to strict state control were tolerated. Because of the communists' goal to eradicate traditional religion as what they perceived to be an irrational belief system, powerful religious groups such as the Catholic Church were among the strongest enemies of communism since its very inception. See also
China mounts 'atheist propaganda' drive in Tibet. Communist doctrine aside, many dictatorships have regulated or forbidden religious groups which were viewed as possible centers of opposition against their totalitarian rule. On the other hand, western intelligence agencies have often cooperated with local religious groups in order to build up opposition in hostile countries (an extreme example being the training and funding of the radical fundamentalist Mujaheddin in Afghanistan by the CIA in the 1980s).
Notwithstanding Cold War attitudes, atheists are legally protected from discrimination in the United States and they have been among the strongest advocates of the legal separation of church and state. American courts have regularly, if controversially, interpreted the constitutional requirement for separation of church and state as protecting the freedoms of non-believers, as well as prohibiting the establishment of any state religion. Atheists often sum up the legal situation with the phrase: "Freedom of religion also means freedom from religion." 
In early 2004, it was announced that atheism would be taught during religious education classes in Britain. A spokesman of the 'Qualifications and Curriculum Authority' stated the following about the decision: "There are many children in England who have no religious affiliation and their beliefs and ideas, whatever they are, should be taken very seriously." There is also considerable debate in the UK on the status of faith-based schools, which use religious, as well as academic, selection criteria .
A 1995 survey  attributed to the Encyclopedia Britannica indicates that non-religious are about 14.7% of the world's population, and atheists around 3.8%.
A 2004 survey by the CIA in the World Factbook  estimates about 12.5% non-religious and about 2.4% for atheists.
In the 2001 Australian Census  15.5% of respondents ticked 'no religion' and a further 11.7% either did not state their religion or were deemed to have described it inadequately (there was a popular campaign at the time to have people describe themselves as Jedi).
A 2002 survey by Adherents.com
 estimates the number of "secular, non-religious, agnostics and atheists" as about 14%.
A 2004 survey by the BBC  in 10 countries showed the proportion of "people who don't believe in God nor in a higher power" varying between 0% and 30%, with an average close to 10% in the countries surveyed. About 8% of the respondents stated specifically that they consider themselves atheists.