This question gets into some technicalities of DNA and RNA that we see often come up in biology. It turns out whether it's a nucleotide or a nitrogenous base weighs heavily on the molecule's function in the body. In fact, there is a category between these two called "nucleoside" even!
Here are the differences.
The most basic unit is the nitrogenous base. It is what undergoes base-pairing in DNA and (sometimes) in RNA that allows these molecules to retain their tertiary/quaternary structures. This base is the adenine, guanine, thymine, uracil, cytosine.
The next unit up is the nucleoside. Now, the base is boned to a sugar at a very specific point on the base and sugar. Depending on whether the sugar is fully hydroxylated or not is the determining factor in whether the nucleoside is going to be in DNA or RNA. Now, nucleoside analogues to the bases above are (outside parentheses = RNA, inside parentheses = DNA): adenosine (Deoxyadenosine), guanosine (Deoxyguanosine), 5-methyl Uridine (Thymidine), Uridine (Deoxyuridine), Cytidine (Deoxycytidine).
So, so far, an easy way to differentiate between the bases and the nucleosides: if it has a "d" near the end, it's a nucleoside. Otherwise, it's the nitrogenous base.
Now, moving on to nucleotides. These big guys are just going to be nucleosides with added-on phosphate groups. This group includes such important energy-producing molecules as ATP and GTP. These are REALLY easy to identify, becaues they all have mono-, di-, or triphosphate in the name! Here are some examples analagous to the bases and nucleosides we talked about: adenosine triphosphate (ATP), Guanosine diphosphate (GDP), Thymidine triphosphate (TTP), Uridine monophosphate (UMP), Cytidine triphosphate (CTP).
Just a recap:
- if it ends in "-dine" or has a "d" in the ending, it's a nucleoside
- if it has "mono/di/triphosphate" in the name, it's a nucleotide
- if it doesn't have either of these, it's just the nitrogenous base.
Base + Sugar = Nucleoside (S for Sugar Only)
Base + Sugar + Phosphate = Nucleotide (T for ATP, the main one)
I hope that helps!