Sodom and Gomorrah were some sinful cities. There was actually three more cities, which were part of a coallition called the Pentapolis. Adamah, Zeboim, and Zoar where the names of the other cities.
The book of Genesis tells us the people in these cities were extremely sinful in God's eyes. And it wasn't just a part of the population there that was sinful. The entire way of life in the cities was very vile. When God told Abram that he was going to destroy the city, Abram petitioned God to spare it. He pleaded God to not destroy the cities if there were 50 righteous people there. God said he wouldn't do it if He found 50 righteous people there. Both Abram and God knew there is no way those cities would reach the 50 righteous people mark though. So Abram kept lowering the number. He got God to promise to not destroy the cities if He could find 10 good people there. But it turns out the cities didn't even have that. The only righteous person God could find in the city of Sodom, was Abram's relative Lot, who moved out of Abram's house a few years back.
The reason Lot chose to move there, was because it was a prosperous place to live. The book of Genesis tells us the area had a good amount of percipitation. It was a land full of lush vegetation like the garden of Eden, or the land of Egypt, in the ancient days. And good percipitation leads to good economic conditions for the people.
So the residents of the Pentapolis knew little worry. And they spent a lot of time partying.
Here's what the book of Jasher has to say about Sodom and Gomorrah.
A few times a year, the cities held region-wide festivities. This was a big party spanning several days, where everybody got drunk, and slept around with everybody. Fathers and husbands looked at their daughters and wives with another man, and didn't care. They were too busy doing the same thing.
A lot of people say the Sodomites were homosexual. This is deduced from the passage in Genesis when the angels came to Lot and the inhabitants of Sodom came and wanted to "know" the men who came to his house. But homosexual is not even an accurate description for their sexual orientation. It wasn't just a few grown men who came to molest Lot's guests. It was an entire city from old to young. It was like hippies from the American 60s walking around with constant perverted thoughts, (God bless the hippies), except it didn't matter what gender they were attacking, and everybody was a hippie.
When archaelogists dig up stuff around the Sodom area, they wouldn't show it on TV, because it wouldn't be appropriate for all audiences.
Besides their perverted lifestyle, the cities were simply a physically dangerous territory for the surrounding peoples. What Lot did by inviting weary strangers into his house and providing them food and rest, was actually against the law in those cities. Anybody who was found to show any kindness to a stranger was punished by death. If a stranger tried to appeal to court, the court found the stranger guilty of traspassing, and officially ruled in favor of execution.
When a stranger entered the city, the Sodomites would greet them and kindly welcome them, even give them a sum of gold to spend on buying things in their city. But they would not let them leave the city. Nobody was allowed to sell anything to them, or give them food or water. So the visitor would roam around the city until they starved to death. Then the Sodomites would take their gold back and steel all valuables the visitor may have brought with them.
There's a story about Lot's other daughter, who did not leave the city with Lot. One time she saw one such stranger walking around the city and nobody would give him anything to eat or drink. She felt sorry for the fellow, and started to secretly bringing him food and water during the night time. The inhabitants of Sodom kept watching the man, and kept wondering how come he's alive for so long. Then they figured some traitor must be secretly feeding the guy. So they notified everyone to keep a watch out for that traitor. Somebody found out it was Lot's daughter doing the crime. So they caught the girl, poured honey all over her body, and unleashed a colony of bees to sting her to death. This was supposedly the cry that went up to God, after which He decided to put an end to this.
The book of Genesis says that in the valley of Siddim, there were a lot of asphalt pits. When Chedorlaomer of Elam and 3 other kings with him came to reconquer the residents of Pentapolis, they fought in this valley of Siddim. Chedorlaomer defeated them, and soldiers of Pentapolis ran. As they ran, many of them fell into these pits. A good analogy for these pits would be the areas of Yellowstone and Mount Lassen National Parks in the US with sulfuric pits and geysers. So there was some sort of underground geothermal activity going on in that region.
The book of Genesis says God destroyed the four out of five cities by reigning fire and sulfur onto them. (the 5th city was spared thanks to Lot). I don't know what exactly that was. Some people say it was a volcano. I've talked to an archeologist who visits the Jordan area every year to do digs there. They found material in the region which can only form at extreme temperatures. Temperatures beyond what a volcano can produce, or anything else on Earth for that matter. Today, such material is found in regions where nuclear bombs were tested. So it's very likely that the cause of that desctruction was extra-terrestrial. But whatever it was, it is responsible for the existance of the Dead Sea in that area. The Jordan river flows into that region. It flows in, but nothing comes out of it. So if we have an area filled with sulfur, with water flowing into it, over time, a lake full of sulfur would form in the region. And that is what we find there.