Saudi Arabia is an Absolute Monarchy (dictatorship) in the Middle East. Its King (hereditary dictator) is King Salman, the half brother of the late King Abdullah. It also happens to be one of the US's strategic partners. That's mainly because Saudi Arabia has lots of oil and lots of oil revenues, which it has used to buy lots of the US. So they own large amounts of the US economy and are the largest supplier of something that the US is addicted to.
Saudi Arabia has a population of roughly 27.5 Million, covers most of the Arabian peninsula and borders on: Jordan, Iraq, Kuwait, the Persian gulf, the United Arab Emirates, Oman, Qatar. Yemen and the Red Sea. The country is largely desert. 
Saudi Arabia's state religion is Islam and the ruling Islamic sect is the warlike Wahhabi. Saudi Arabia practices Sharia Law.  Most Saudis are Sunni with a Shia minority. Religious toleration only extends to certain sects of Islam, and no other religion is tolerated as such (ancient abandoned churches have been bulldozed despite their archaeological value). Saudi Arabia's late king Abdullah said that he respected rights of other religions to worship in private, but this right is not respected. The Saudi Arabian government has a "dhimmi" system, which gives protection and privileges for non-Muslims but treats them as second-class citizens. Mr Abdullah has said that all religions of the world must unite to fight atheism.
Saudi Arabia includes the cities of Mecca and Medina, the most sacred places in Islam and the origins of the Islamic religion. All devout Muslims who can afford it are expected to undertake Hajj at least once in their life by making a pilgrimage to Mecca. This makes Saudi Arabia very influential in the Islamic world, and gives them control over which Islamic sects are considered to have acceptable differences within Islam or which are deemed heretical.
Saudi Arabia has a Human rights record that is statistically and rather blatantly worse than the Taliban. Women and non-Muslims are regarded as second class citizens. Women are thought to be subject to their husbands, and are not allowed out on their own, or allowed to walk alongside their male companions. Women have to walk six feet behind their accompanying male at all times in public. Homosexuality is punishable by death. So-called 'adultery' (though this is often a scapegoat for religiously and politically conservative prosecutors that have no real case) is punishable by whipping or stoning. A man by the name of Jamal Khashoggi was allegedly killed by the Saudi government.
Human trafficking is a big problem in Saudi Arabia. Thousands of children, men and women live as slaves in Saudi Arabia. Often foreign workers especially women are subjected to sexual abuse and rape. A Saudi who kept an Indonesian women as a slave and used to rape her was allowed to go free from the United States. There have been reports of European and American women kidnapped to be kept as sex slaves in Saudi Arabia.