Although very similar, the base pairs in DNA and RNA differ by one nitrogenous base.
In DNA, the four nitrogenous bases are Adenine, Thymine, Guanine, and Cytosine. Adenine always bonds to thymine (in DNA), they are connected by two Hydrogen bonds. Cytosine and Guanine are always bonded together. They are connected by three Hydrogen bonds.
However, in RNA, there is no thymine and instead, there is Uracil. That is the only nitrogenous base that is different. Cytosine still bonds to guanine, and adenine bonds to uracil.
I hope that helps!