Then the Lord rained upon Sodom and upon Gomorrah brimstone and fire, from the Lord out of heaven. And He overthrew those cities, and all the plain, and all the inhabitants of the cities, and that which grew upon the ground (Genesis 19:24 - 25, HBFV).
People many have also heard the term "fire and brimstone" linked to an especially energetic sermon where the audience is warned that unending torture in an ever-burning Hell awaits all those who do not repent.
What Is It?
The word "brimstone" is recorded 15 times the King James Bible. The book of Revelation uses the word the most with seven occurrences (Revelation 9:17 - 18, 14:10 19:20, 20:10, 21:8).
Brimstone is a 12th century word which literally means "burning stone." In the Old Testament this word is derived from the Hebrew gophriyth (Strong's #H1614) which refers to cypress resin or sulfur. In the New Testament it comes from Greek theion (#G2303) which also references sulfur.
Sulfur is the tenth most common element, by mass, in the universe and the fifth most common element on earth (Wikipedia). It is found in regions with volcanic activity and, not surprisingly, in the valley of the Dead Sea.
Effects
Brimstone, like fire, can kill. This is likely why both of them are the chief ingredients in the lake of fire (Revelation 14:10, 19:20, 20:10, 21:8).
When sulfur burns in the air it produces sulfur dioxide, a toxic gas which can cause life-threatening breathing difficulties within minutes. When sulfur dioxide mixes with water it produces sulfuric acid, a highly corrosive liquid which can cause chemical burns. The fire and brimstone miraculous rained upon the cities of the plain were aided in their destructive powers by igniting pockets of bitumen found in the area (see Genesis 14:10).
Additionally brimstone, like fire, can cleanse. According to Thayer's Greek Definitions, burning it was believed to possess purifying abilities and was utilized to ward off disease. Together they acted to sterilize the land and render it unusable for a period of time (Deuteronomy 29:23).
What Did God Use Them?
The evil behavior God personally witnessed in Sodom and Gomorrah (see Genesis 18:20 - 22) was the same wickedness he saw before Noah's flood.
And the Lord saw that the wickedness of man was great on the earth, and every imagination of the thoughts of his heart was only evil continually . . . And God looked upon the earth, and behold, it was corrupt - for all flesh had corrupted its way upon the earth (Genesis 6:5, 12, HBFV).
Both the pre-flood earth, and Sodom and Gomorrah, had fully dedicated themselves to fulfilling their wicked lusts. Violence was also common as people sought to take, by force, what they demanded (see Genesis 6:13).
Just like the world in Noah's day rightly deserved the death penalty, so too did the behavior of Sodom and its cities. The only difference was that water in the first case, and fire and brimstone in the second, was used to carry out His judgment.
Conclusion
God chose to use fire and brimstone because they could both carry out the death penalty and cleanse the land polluted and trashed by wickedness. Using such agents also left a mark of destruction and desolation in its wake. Sodom and Gomorrah's demise is a living witness that God exists and that all humans will ultimately be held accountable for how they live their lives.