There are good and bad angels. The Satan is the worst of all.
Angels can come to earth when sent on a mission. As angels are only made of the elements of air and fire, it is very difficult for an angel to come to earth, they have to concentrate very hard, to focus themselves here. That is why they vanish suddenly when their mission is complete.
Sometimes only one person will see the angel..... sometimes many will see it. You will not realise it is an angel, it may have taken the form of a human in order to carry out its task. Often people are given tests by angels, appearing as beggars, asking for money or help.
The angels such as those who spoke to Abraham and Jacob were purely spiritual forces which appeared in human form.
Before a baby is born, an angel teaches the baby Torah in the womb:
The Talmud says :
A candle is lit on his head and he is able to see from one end of the world until the other end .... There isn't a better period for a person than these days [in the womb] ... They [the angels] teach him all of Torah ... and as he enters the world, an angel hits him on his mouth and he forgets it [to be recalled later throughout the course of his life through learning]. [Niddah 30b]
For every mitzvah (or good deed) a person does, an angel is created. This angel defends you in the Heavenly Court. And when we sin, or tell lies, or harm others.... we create for ourselves a prosecuting angel.
According to Maimonides, the degree of Divine Providence you experience is directly proportional to your attachment to God, and therefore the commandments which create that attachment are the true "guardian angels" of a person.
Rebbe Meir says: "If a person does one commandment, he is given one angel to guard over him; if he does many commandments, he is given many angels."
The angels that accompany us, watching over us, are our own, the ones we have created with each commandment we do. We might imagine that when we leave this world we'll be accompanied by millions of angels, but our Sages teach us otherwise. We are told that the quality of the angel depends on the quality of our mitzvah. It's a long way to the World to Come, and sometimes a person can arrive there with only one angel because all the others fell by the wayside, too weak to complete the journey. In order to create a powerful angel, we have to do each commandment "like fire," in Hebrew, K'AiSh, K = Kavanah (intent), A = Ahavah (love), and S = Simchah (joy), with deep intent, love, and joy.
Everything is by Divine Providence. If a leaf is turned over by a breeze, it is only because this has been specifically ordained by G-d to serve a particular function within the purpose of creation. [Ba'al Shem Tov]
The Talmud teaches that in many matters G-d controls the world through intermediaries (angels). In fact, the Talmud states that there are only three keys that G-d didn’t give over to intermediaries. They are: the Key of Rain (parnosa/sustenance), the Key of Childbirth, and the Key of Techias HaMaisim (the Revival of the Dead).