The Earth (and the other planets in the Solar System) formed by the accretion of gas and dust out of the protoplanetary disk.

http://cosmos.colorado.edu/astr1120/l9S4.htm

At the very end of this process, there were around 10 large planets -- more or less what we have now in the Solar System -- plus a large number of smaller objects called planetesimals. Planetesimals are (were) like very large comets and/or asteroids which could easily have been 10s to 100s of kilometers in diameter.

It probably took something like 10s of millions of years for the planets to form. Near the end of this time, the Moon was created through a giant impact with the Earth.

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