The post hardcore genre developed from the hardcore scene in the USA in the late 1970's. Post harcore is based on the same structure as hardcore but with a few added creations such as beatdowns, blast beats and breakdowns.
The beatdown consists of a standard 4/4 rock beat which is played with a lot of aggresion so the song feels really powerful and edgy, the guitars are usually play chords or moderatly fast picking during this part of the song,the vocals in this part of the song could be screaming or singing.
A blast beat is usually used a pre-chorus or can be used as an intro or build up to a solo. for a blast beat the drums would be playing double bass while the snare and hi-hats would be played alternatly or two strikes on the snare for every one on the hi-hat at high speed while playing moderatly fast double bass, the guitars in this part of the song would be playing very fast alternate picking, the vocals during a blast beat are usually a very deep scream or roar.
In post hardcore the breakdown is probably the most powerful part of the song because this is generally the heaviest part. In a breakdown the drums would slow down into half time or even slower and would play a pattern on the bass drum usually with a lot of double which can be fast or slow which the guitarists would follow, the guitarists would follow the double bass pattern playing palm muted notes on the lower strings to make it really heavy. The vocals in a break down tend to go from a period of low screams to a period of higher screams which may be done by the one vocalist or going between two vocalists.
Some bands in this genre are
Here are some good post hardcore bands: I Killed The Prom Queen, Parkway Drive, As I Lay Dying, Norma Jean, Bleeding Through, Every Time I Die, Bring Me The Horizon, August Burns Red, All Shall Perish