Answer to Question 1
Terrestrial planets are small, dense, rocky worlds with little or no atmosphere. Jovian planets are large, low-density worlds with thick atmospheres and liquid or ice interiors.
Answer to Question 2
The outer four planets, Jupiter, Saturn, Uranus, and Neptune, are Jovian (Jupiter-like) planets, meaning they are large, low-density worlds with thick atmospheres and liquid or ice interiors. According to the solar nebula theory, far from the Sun, beyond the ice line, compounds such as methane and ammonia could condense to form other types of ice. Water vapor, methane, and ammonia were abundant in the solar nebula, so beyond the ice line the nebula would have been filled with a blizzard of ice particles, mixed with small amounts of silicate and metal particles that could also condense there. Those ices are low-density materials. The densities of Jupiter and the other outer planets correspond to a mix of ices plus relatively small amounts of silicates and metal.