When it comes to defining a subgenre of punk, post-hardcore is really not much more than a vague catchall, set up to encompass any band that has taken their musical roots from hardcore, but expanded their sound.
Post-hardcore really began in the ‘80s, with the actual hardcore bands themselves. As bands like Black Flag began to bore with the formulaic constraints of hardcore, more experimental sounds began to appear in their music. Just as when the Clash broke away from traditional punk rock by adding elements of reggae to their music, hardcore bands were adding elements of jazz, noise rock, prog rock and math rock to their sounds.
Additionally, this era saw Rites of Spring, often considered the earliest incarnation of emo, another subgenre that gets lumped in with post-hardcore.
The ‘90s saw emo begin to rise to prominence, with experimental bands like Hot Water Music and At The Drive-In. At the same time, the waters of post-hardcore were getting even muddier as a sound description, as bands like Fugazi, who added funk and dub beats and glam rock riffs, and Glassjaw, who added whatever they liked, were also drawing from hardcore influences and doing things that were even more withdrawn from traditional hardcore punk.
In the new millennium, post-hardcore is even more vague. Denoting a band a post-hardcore does little to describe their sound at all. Post-hardcore can now refer to bands that are emo, screamo, experimental or even pop punk, as long as elements of the heavier roots of hardcore are present in their music. (For example, all emo is post-hardcore, but not all post-hardcore is emo.)
Many of punk’s most popular bands today can be called post-hardcore, but while they share many of the same influences, and often the same penchant for screaming, the definition of post-hardcore as a sound is getting less and less definitive by the day.