Originally Posted by WhateverDude

Screamo's sort of a subgenre of post-hardcore. Like hardcore is a subgenre of punk.

Some screamo takes more influence from post-hardcore, so you'll have more intricate song structures and instrumentation. Weird time signatures. Build-ups, etc. Some screamo takes more from hardcore, just blasts through at breakneck pace with pretty straightforward instrumentation and structure. Plenty of bands do a bit of both.

Post-hardcore tends to be more bands like Fugazi, Jawbox, Bear vs. Shark, Pile. Vocals are generally more yelly or sung.

Screamo is more bands like Orchid, maybe Rights of Spring, Indian Summer (who definitely take a huge influence from Post-HC), Swing Kids...

Those are the heavy hitting, big names of the genres.