There are a number of different types of angels, most notably in Christian mythology, which has a complex hierarchy of angels. Many people believe that religions which include angels owe the presence of angels in their faith to Zoroastrianism, where a complex and varied cast of angels and demons plays an important role in religious faith. In any case, angels are generally viewed as divine beings who enjoy the blessings of God, acting as intermediaries between Gods and men.
Beliefs about angels are surprisingly consistent across many cultures. For example, they are often linked with fire and light, suggesting both the wrath of God and the light of faith. They are often visually depicted with wings and halos, and they are generally believed to be quite beautiful, although they are supposedly invisible to humans. Many religions also include the concept of a guardian angel, who looks after people and guides them through their lives.
In Christianity, the big kahunas are the seraphim, cherubim, and thrones, who interact directly with God. The seraphim are fiery beings of light, while cherubim look nothing like the plump cupids you see on Valentine's cards; rather they are fearsome being with multiple heads who guard the stars on God's orders. The thrones, also called wheels or ophanim, are wheels of fire, covered in many eyes.
In the “second sphere” of Christian angels, there are an assortment of angels who act almost like administrators, including virtues, dominions, and principalities. Among the lower ranks are powers, archangels, and simple angels. According to Christian belief, angels interact directly with humans, looking after them and assisting them in times of need. You may be familiar with some of the archangels, God's messengers, such as the Archangel Gabriel.
In Islam, angels are known as malaaikah, and they have no free will, being created expressly by God to worship and glorify Him. This creates an interesting dichotomy, as demons in Muslim mythology do have free will, which allows them to defy God and engage in a number of unsanctioned activities.
Many other religions have some beliefs about benevolent or friendly spirits, although they may not be known as angels. Just as there is a universal idea to explain evil with the use of demons, the widespread belief in friendly spirits seems to suggest a desire to know that there are heavenly figures to look after us on Earth, even if we cannot interact with them directly.