Here's a breakdown of what a nucleotide is:

* Sugar: A five-carbon sugar (either ribose for RNA or deoxyribose for DNA).

* Phosphate group: A molecule containing phosphorus and oxygen.

* Nitrogenous base: A molecule containing nitrogen that acts as the "information carrier." There are five main nitrogenous bases: adenine (A), guanine (G), cytosine (C), thymine (T) (in DNA), and uracil (U) (in RNA).

Nucleotides link together to form long chains, which are the building blocks of DNA and RNA.