The ACLU was founded in 1917 by Roger Baldwin and was made for the purpose of protecting Americans from the injustices inflicted by the Conservatives, who were very much in power at the time. They defend those whose liberties are under attack regardless of which side of the political spectrum. Even today, they continue to defend our freedoms from Neocons and regular Cons.
Conservatives hate the ACLU because of its secular agenda, even though separation of the church and state is guaranteed, even though they've sued organizations on behalf of people who wanted to be able to do things like wear anti-abortion T-shirts and use a prison chapel Clearly, this shows what they really think of freedom.
The ACLU works to improve the position of people that Conservatives have traditionally left without their rights when they could. That includes, gay men and women, ethnic minorities, prisoners and disabled people. The ACLU feels that weakening the rights of the most vulnerable potentially weakens everyone.
The ACLU fights in courts, and campaigns to get laws enacted of preserved ensure what they see as the freedom and rights that the Constitution should preserve.
Your First Amendment rights - freedom of speech, association and assembly; freedom of the press, and freedom of religion. Your right to equal protection under the law - protection against unlawful discrimination. Your right to due process - fair treatment by the government whenever the loss of your liberty or property is at stake. Your right to privacy - freedom from unwarranted government intrusion into your personal and private affairs.