In this video I quickly go over the structure of the DNA and RNA bases and nucleotides, and give some mnemonics for how to remember them.
Molecular structure of DNA and RNA.
The difference between a nucleotide, nucleoside, and base.
nucleotide: base, sugar, one or more phosphates
nucleoside: base and sugar
base: just the base
RNA uses uracil (U) instead of thymine (T). The difference is a methyl group that doesn’t affect base pairing.
The ribose sugar of RNA has a hydroxyl group at the 2′ position that the deoxyribose of DNA lacks. RNA is less stable because of the extra hydroxyl.

The GACT and GACU mnemonics for the DNA and RNA bases
GA: purines
CT/CU: pyrimidines
G base pairs with C
A base pairs with T/U
GC base pairs have 3 hydrogen bonds
AT and AU base pairs have 2 hydrogen bonds, and so are a little weaker.