The inner planets -- Mercury, Venus, Earth and Mars -- are the rocky, metallic, major planets which formed near the Sun. Today we refer to them as terrestrial planets since they are much like Earth.
With Earth in the midst of the inner planets, they have been explored more thoroughly than the outer planets.
There is a natural dividing line between the inner planets and the outer planets -- the Asteroid Belt. It probably is composed of smashed planetesimals, clumps of rock and other matter floating around in space as the Solar System formed. The Asteroid Belt is in an orbit farther from the Sun than Mars, but not as far as Jupiter, which is the first of the outer planets.
