It is generally understood that angels were created by the Source before human were added to the universal scheme.  When we were created, the lower orders of angels were invited to look after us.  Guardian angels come from the lowest ranks of the angelic hierarchy and their evolution depends on ours.
Opinion varies about the various orders of angels, but it is generally considered that they are made up of three hierarchies with three levels in each.

The first and highest sphere consists of the Seraphim, Cherubim and Thrones.
The Seraphim, Cherubim and Thrones receive direct illumination from Source being “GOD”.  They transform and step down the light of God to a level where it can be accepted by the lower orders in the Universe without burning them out.

Seraphim
These are the highest order of angels.  Their essence is pure love and they are the heavenly hosts who surround the blazing throne of the Godhead.  They constantly sing the praises of the Creator, thus maintaining the vibration of creation.  They direct the divine energy as it emanates from Source.

Cherubim
These are the angels of wisdom who spread the ineffable light of Source.  They are the guardians of the stars and the heavens.

Thrones
The Thrones look after and protects the planets.  There are several Thrones looking after our planet now because it is important in the universal scheme that Earth ascends to a higher way of living. 

The second sphere is made up of the Dominions, Virtues and Powers.

Dominions
The Dominions are like celestial prefects.  They have the responsibility of overseeing those under them in the angelic hierarchy.  They are channels of mercy.  Although the Dominions rarely make direct connections with humans they help to smooth the passage between the spiritual and material worlds.

Virtues
The Virtues transmit light in a form which we humans can receive.  When you tune into the angels these are the ones who make miracles happen.  They hold a pool of spiritual information which we can collectively access to bring in the new Golden Age.

Powers
From this order come the Angels of Birth and Death.  They are present at all births and come to guide all transitions to the spiritual planes at death.  They help those humans who are lost in the astral planes after leaving their bodies.

The Lord of Karma, who guard the conscience of human beings, are the Powers.  Every thought we have, word we speak or action we take impacts on our life.  It also affects other people and the Universe.  If our emanations are positive, we earn good karma.  If negative, we attract bad karma.  Our karma, the balance of good and bad continues throughout our soul’s journey.  Bad luck comes when our soul decides to repay accumulated bad debts, possibly from former incarnations.  Good fortune smiles on us when we draw on karmic savings.  No one ever gets away with anything.  There is a spiritual working out of all things and the Lords of Karma administer this.  They are in charge of the karmic, or akashic records.  Before you incarnate they help your Higher Self evaluate your past lives on Earth as well as your experiences in the other galaxies.  Together you make choices about what you need to learn and do during your current life.

The third and lowest sphere is comprised of the Principalities, Archangels and Angels

Principalities
Principalities look after and protect cities, nations, multinational corporations and any very large structure of people.

Archangels
Archangels are in charge of the angels.  They mostly oversee vast projects for the light.  There are millions of Archangels throughout the cosmos.  Those most closely connected with Earth are Archangel Michael, Archangel Gabriel, Archangel Chamuel, Archangel Jophiel, Archangel Zadkiel, Archangel Uriel and Archangel Raphael.  Archangel Metatron is now helping us much more on our planet.
The Archangels have retreats located in the etheric around the Earth and we can ask to go to them in meditation or dreamlike state for any help we may need.

Angels
Angels are our guardians on Earth.  Our personal guardian angel also keeps our individual akashic record up to date.  They love us unconditionally and our lives transform when we connect with them and allow them to help us.
The hierarchy of angels works throughout the Universe and Earth is a very small part of their domain.