Short answers, pulled from the story.
Ray Bradbury's A Sound of Thunder establishes the defining metaphor that a single tiny change in the past can ripple forward to create a completely different future. The story illustrates how history is fragile and that the smallest deviation can lead to a world unrecognizable from our own. This concept of the point of divergence remains the engine of the genre.
The first work of fiction detailing an alternate history was Joanot Martorell's 1490 epic romance Tirant lo Blanch. This story told the tale of a knight who saved Constantinople from Islamic conquest. It laid the groundwork for a genre that would eventually become a staple of speculative fiction.
The Usenet Alternate History List was first posted on the 11th of April 1991 to the Usenet newsgroup rec.arts.sf-lovers. This early online community allowed fans to showcase their own works and provide useful tools for those searching for anything alternate history. Its prominence later declined with the migration to moderated web forums.
Philip K. Dick's The Man in the High Castle published in 1962 depicts a divided United States where the Empire of Japan takes the Pacific states and Nazi Germany takes the East Coast. The story reflects the cultural anxiety of the Cold War era regarding the threat of nuclear annihilation and totalitarian regimes. It became a common trope allowing authors to examine the darkest possibilities of human history.
Ward Moore published Bring the Jubilee in 1953 featuring a protagonist who lived in an alternate history where the Confederacy had won the American Civil War. The story involves a time patrol where guardians moved through time to preserve the correct history. It introduced the notion of a time patrol as a theme that would become a staple of the genre.