These are all chemical compounds containing calcium.

Ca stands for calciumCaO is calcium oxideCa(OH)2 is calcium hydroxideCa(NO3)2 is calcium nitrateCa3(PO4)2 is calcium phosphate

Each of these compounds has different properties and uses. Calcium is an essential mineral for the human body and is important for bone health, muscle function, and nerve function. Calcium compounds are also used for various industrial purposes, such as in construction materials and fertilizers.