Planets can be classified in several ways. For example, they can be classified by size (giant and small), by position relative to sun (inner planets and outer planets), by history (classical and modern), and by composition (terrestrial and gas planets). Classification by composition gives us terrestrial planets (Mercury, Mars, Venus and Earth) and gas planets (Jupiter, Uranus, Saturn and Neptune). Since the list of terrestrial planets is similar to inner planets, both terms are often used interchangeably. They can also be referred to rocky planets.
Basically, rocky planets are composed primarily of metal and rocks. They have substantial densities, solid surfaces, are relatively small, are ring-less, and have one/more than one natural satellite(s). The four planets; Mercury, Venus, Earth and Mars orbit closest to the sun and have distinctly different characteristics from those that orbit far from sun (Saturn, Uranus, Jupiter and Neptune). Though each of these planets is different, they share similar characteristics up to a certain degree. The planets that orbit far from sun are also known as gas giants because they contain very high gaseous properties and lack solid surfaces. Among the four rocky planets, only Earth is known to have living inhabitants (human and other species). The fact that Earth contains oxygen and an active hydrosphere contributes to the sustainability of living things. The surfaces of inner planets are marked by impact craters and tectonic properties (i.e. volcanoes).
Many people seem to equate the term “inner” with “inferior” planet. The latter describes planets which are closer to the sun than Earth is; Venus and Mercury. In terms of average distance from the sun among rocky planets, Mercury rates closest with 57,900,000 kilometers away while Mars is about 227,900,000 kilometers away from the sun.
Rocky planets are grouped as such because they share similar characteristics in terms of composition. To find out more about each of these planets, you can gather information from various resources (i.e. book, magazine, and internet).