A political spectrum is a classification scheme that is used to compare different political philosophies based on where they lie on the spectrum. A spectrum is defined by its axis (or axes, as some spectrums have more than one). Usually there is one issue that is deemed essential, and which is thought to subsume all others; placement on the spectrum is determined by one's position on that issue.
Because there are many possible political spectrums, one should not talk about _the_ political spectrum as if there were only one. At the very least, one should first establish context by defining the axis upon which different positions will be measured. Unfortunately, this is seldom done (see Politics, e.g.).
In a modern Islamic country, the political spectrum might be divided along the issue of the clergy's role in government. Those who believe clerics should have the power to enforce Islamic law are on one end of the spectrum, those who support a secular society are on the other.
In modern Western countries, the spectrum is usually defined along an axis of Conservatism ("the right") versus Socialism ("the left", called Liberalism in the United States). There are different opinions about what is actually being measured along this axis. Some people view it as a measure of social equality, some as a measure of the government's role in the economy, some as a measure of religion's place in society, some as a measure of the different weight put on fair outcomes versus fair processes. Eric Hoffer suggested that the right-left spectrum measures whether one is frightened by change (the right) or whether one embraces change (the left). Thomas Sowell has argued that one is left-wing to the extent that one believes human nature, and therefore human society, is malleable, and right-wing to the extent that one believes human nature is fixed. Since it is not obvious how these various concepts are related, it is very confusing to speak of the right or the left without indicating what exactly you are refering to.
Nonetheless, the right-left spectrum is so common as to be taken for granted. Many people have a hard time conceptualizing any alternative to it. However, numerous alternatives exist, usually having been developed by people who feel their views are not fairly represented on the traditional right-left spectrum.
Such an alternative, and an example of a two-axes spectrum, has been popularized by the Advocates for Self-Government. One axis measures your views on "personal self-government", which pertains to the government's role in the home, church, and between consenting adults in private, while the other axis measures "economic self-government", which pertains to the government's role in economic activities. With two axes, there are four different extremes: Libertarian, which favors personal freedom and economic freedom; Left Liberal (or Socialist), which favors personal freedom but opposes economic freedom; Right Conservative, which favors economic freedom but opposes personal freedom; and Authoritarian, which opposes both personal freedom and economic freedom. There is, of course, plenty of room in the center for people who hold moderate views along either axis.
Another alternative currently popular among certain environmentalists uses a single axis to measure the good of the earth against the good of big business, which is seen as being the force most likely to harm the earth. On this axis, many mainstream politicians normally considered left-wing (such as Bill Clinton) are considered no different from those normally considered right-wing, because of their allegedly pro-business policies.
In 1998, political author Virginia Postrel, in her book The Future and it's Enemies, offered a new single axis spectrum that measures ones view of the future. On one extreme are those who allegedly fear the future and wish to control it. On the other hand are those who want the future to unfold naturally and without attempts to plan and control.