Screamo is a musical genre which evolved from emo and hardcore punk in the early 1990s. Characteristic of the genre are screaming vocals, harmonized guitars, and fast-paced riffs. Screamo has been described as "mixing the literate, poetic lyrics of emo punk with a harsher and more metallic brand of sonic thrash" as well as using screaming vocals "as a kind of crescendo element, a sonic weapon to be trotted out when the music and lyrics (every bit as evolved and autobiographically sincere as emo’s were) reach a particular emotional pitch". Other than that, it is fairly hard to classify (particularly since the rule about screaming vocals is bent fairly often). In addition to that, screamo has significant overlap with other genres like grindcore and even post rock.
Or so says Wikipedia.
THIS is screamo: