Angel wards are enochian-based symbols that prevent the entry of an angel from a certain location. The wards vary in appearance. Alastair, a demon, used complex symbols inscribed in circles. The runes he used were invisible to the unaided eye. Hunters and Dr. Visyak, a purgatory native, used sigils that look more simplistic, drawn with chalk, or a blood-like substance. Crowley used runes posted on the four general directions of a building, and required disabling all four before an angel could enter the building. Crowley complemented the wards with other symbols that prevented Castiel from reaching his full strength.
Writing over the sigils disable their effects. Additionally, drawing the runes need meticulous attention to detail. If even one is drawn or arranged improperly, all of them would not work at all, as seen when Castiel was able to infiltrate Bobby's house despite being laden with wards.
It is also shown in "Road Trip" that enochian warding magic can prevent Angels from listening to a conversation from anyone in the warded room.