1) A nucleoside (with an s) consists of a nitrogenous base covalently attached to a (ribose or deoxyribose) sugar but without the phosphate group.
2) A nucleotide (with a t) consists of a nitrogenous base, a sugar, and a phosphate group. So, a nucleotide is a “nucleoside mono-phosphate.”
3) A nucleic acid contains a chain of nucleotides covalently linked together to form a sugar-phosphate backbone with protruding nitrogenous bases. In RNA (ribonucleic acid), the sugar groups are ribose, whereas in DNA (deoxyribonucleic acid), deoxyribose sugars are present instead of ribose.
“Nucleic Acid VS Nucleotide,” Newton; accessed on april13,2013. http://www.newton.dep.anl.gov/askasci/mole00/mole00296.htm.