Uracil is found in RNA, the DNA-like molecule involved in copying genetic information for translation (mRNA) as well as making up several of the participants in translation (tRNA, rRNA). Its pairing partner is adenine, with which it forms two hydrogen bonds.

Uracil (right) shown in pairing orientation with adenine (left). The red and blue halos represent partial negative (red) and positive (blue) charges that lead to base pairing. 3D view.
Uracil is noteworthy because it's the 'different' base between DNA and RNA. In DNA, adenine's partner is thymine, taking the place of uracil. There are many misconceptions about the likely reason uracil was replaced by DNA. The first point to notice is that uracil and thymine are identical in their partnering interactions with adenine, and neither is meaningfully more stable as a chemical. The ONLY chemical difference is that a thymine is a uracil with a methyl (-CH3) group added relatively distant from where basepairing takes place.
Thymine vs. uracil. The only difference is the methyl (-CH3) group on thymine, circled in yellow. (Note that the orientation of the molecules is different than in the figure above). 3D view.

Frequently Asked Questionsedit

Why did evolution 'switch' to the use of thymine for every DNA-using organism we know of?edit

The answer is a bit complicated; it actually starts with cytosine. Cytosine can undergo a very common chemical reaction with water that results in changes to two of its basepairing positions. However the shortest summary of what happens is that this reaction changes cytosine to uracil. Literally. There is no chemical difference between a 'real' uracil and a cytosine that has had a chance interaction with water and become uracil.

How does DNA using thymine instead of uracil help?edit

Treating the bases as letters helps: in RNA, the code letters are A, G, C, U. If a C => U change occurs, then nothing 'looks' wrong to the machines inspecting the RNA molecule for errors—uracils are normal parts of the code. But in DNA, code letters are A, G, C, T. If a C becomes a U, it's instantly obvious—there are no (legitimate) "U's" in DNA. On this basis, mutations arising from C => U mutations can be 'spotted' and prevented in DNA... but in RNA, there's nothing that can be done.