The Angel Arm is a natural ability found in all Plants. Capable of a wide variety of forms, these Angel Arms are most commonly known for their usage by Vash and Knives in which they appear as enormous cannons with the faces and wings of angel's (thus their name). They're capable of great destruction in a manner similar to that of an atomic bomb.
In the Anime, individual Plants require a catalyst for the Arms to appear, such as the two pistols made by Knives. The destruction of July is how Vash earned the name "The Humanoid Typhoon". After the use of the Angel Arm in Augusta and July, Vash is said to only have fragmented memories in regards to the blast.