-Aiya Sensei... That hurt! Well what just gave me a bruise is a rock made of the earth. This is important to us ninjas because it is one of the basic 5 elements or chakra nature manipulation.
The strengths and weaknesses:
It is superior to Water because water flows towards the rocks and the Earth yet cannot break it. Also, water added to earth just makes mud, it does not destroys it. So jutsu wise, an S-Rank Water = A-Rank Earth.
It is inferior to Lightning due to the fact that lightning cracks through the insides of the earth enabling it to destroy it completely. So jutsu wise, S- Rank Earth = A-Rank Lightning.
It is neutral with the other elements Wind and Fire, fire having no special effects to Earth and vise versa. Wind however can be used in conjunction to hurl rocks, earth, or boulders as such.
Also, Earth can be combined with other elements to make a new one. For example, Earth and Water= Mokuton.