Enochian sigils belong to a type of magic that originated from the Enochian language. Both demons and angels have knowledge of the sigils, although the use originated from the latter. This branch of magic is heavily reliant on use of runes derived from the eponymous language of the angels. Hunters have some knowledge of it, but do not completely understand it.
There are many uses for Enochian sigils:
Sometimes referred to as "angel-proofing", these sigils can keep angels out of a building. They can be invisible to the human eye, but angels and spirits can see them. They are mainly cast by demons. This technique is first seen in Death Takes A Holiday, when Sam and Dean noticed glowing runes painted all over the funeral home. Castiel later confirms to Dean that those runes keep angels out. Crowley had Enochian sigils all over his mansion which prevented Castiel from entering (Abandon All Hope...).
Sam and Dean are later forced to write them all over Bobby's house to keep Castiel out in The Man Who Would Be King - though Bobby did not do a very good job. They get it right in The Man Who Knew Too Much.
One powerful angel ward appears in The Vessel. The ward is an enochian symbol that, if drawn on an object, even a large submarine, can protect it. It can even repel archangels. The spell was intricate and tied to the life of the caster, so even removing the sigils doesn't stop the spell.
Concealment From Angels
|“||They're Enochian sigils. They'll hide you from every angel in creation, including Lucifer."
"What? You just branded us with it?!"
"No, I carved in to your ribs.
— Castiel and Dean
in Sympathy for the Devil
An intricate series of sigils 'carved' in the surface of a person's ribcage. With them, humans can hide from literally all angels, including archangels, and can't be found. So far, no angel has been able to overpower this; however, Lucifer eventually finds Sam in a dream, telling him, "You're a hard one to find, Sam". But, Metatron, as God's scribe, could erase them if he found them, and even gods said it would be easier to break their ribs than scrub the symbols off. Dean, Sam, and Adam are the only humans known to have these carved into their ribs (it's possible that Bobby also had it done). This was first seen in Sympathy for the Devil when Castiel carves them into Sam and Dean's ribs to hide them from the angels. It even hides the recipients from the angel responsible for bestowing it to them, and as such, Castiel needed to contact the Winchesters via phone and vice-versa in order to pinpoint their location. However, this concealment can easily be bypassed by angels via dream walking, though doing so does not allow them to discern the location of the dreamer, unless the dreamer willingly reveals to the angel his or her actual location. (Point of No Return)
This is seen in Frontierland, when an injured Castiel paints a sigil on Bobby's fridge to hide himself while he healed. It is believed that it hides an angel's presence from any other angel.
Castiel later puts some sigils up in chalk on Rufus' Cabin to protect them from his fellow angels. These are rendered useless when Meg goes out to kill two demons and thus exposes their location, though the sigils are seen in later episodes.
Castiel, in order to hide from the angels that are after him, has an Enochian sigil tattooed on his stomach. (I'm No Angel) However, this doesn't protect him from an angel who has special abilities such as Ephraim who is able to locate him through his pain. (Heaven Can't Wait)
Demon Body Imprisonment
When sigils are carved into the human the demon is possessing, they lock the demon inside the human's body, making it unable to leave. This is first seen in The Devil You Know when Crowley carves them into Brady's chest to stop him from escaping.
Suppressing the Power of Angels
Enochian Sigils are capable of leaving most Angels powerless, with only Lucifer showing the ability to overcome it. In addition, a specific sigil has been seen to affect even an Archangel as powerful as Lucifer, significantly weakening his powers.
Perhaps the most powerful of any form of Enochian Sigils are used in Lucifer's Cage. The entire Cage has been seen to be covered in Enochian runes. They must be extremely powerful, having bound not only Lucifer, but Michael as well, completely, leaving them unable to do anything to leave. It was only because of the Darkness's release damaging the Cage that Lucifer was able to reach out to Earth but even then he still could not leave.