The people of this city, along with the immediately surrounding towns and villages (Genesis 19:25) were destroyed when the Lord rained down burning sulfur upon them. Subsequently these cities came to serve as symbols throughout the Bible of divine judgment executed by fire (Isaiah 1:10; Ezekiel 16:46; Zephaniah 2:9; Matthew 11:23; Revelation 11:8). In the context of Jude, he is making the point that just as these men and women were judged for their sins with fire, in like manner the heretics will also be judged by God.
7 just as Sodom and Gomorrah and the surrounding cities, which likewise indulged in sexual immorality and pursued unnatural desire, serve as an example by undergoing a punishment of eternal fire.