Uracil is the RNA base that replaces thymine.
RNA and DNA both use adenine, cytosine, and guanine, but RNA generally uses uracil as its fourth base. Uracil pairs with adenine during transcription and in RNA molecules that form double-stranded regions.
DNA uses thymine instead. Thymine is chemically more stable in DNA and helps cells detect some forms of cytosine damage, because spontaneous deamination of cytosine produces uracil. Finding uracil in DNA can therefore signal damage requiring repair.
Uracil is also present in uridine triphosphate, or UTP, which cells use to build RNA. The distinction between uracil and thymine is a useful way to identify whether a molecule is RNA or DNA, although some unusual organisms and modified nucleic acids have exceptions.