RNA transcription defines the process by which DNA is converted into RNA. In eukaryotes, RNA transcription takes place in the nucleus.
An enzyme called RNA polymerase (RNAP) facilitates transcription by traversing one strand of DNA (in the 3'-5' direction), called the template strand. At each successive base, RNAP adds the complementary base to the growing strand of RNA, where uracil is substituted for thymine.
Because the RNA strand created (called pre-mRNA) is constructed on complementarity, it is identical to the opposite DNA strand (except for the replacement of thymine with uracil). Accordingly, the second DNA strand is called the coding strand, because even though it is not used during transcription, the coding strand is identical to the pre-mRNA strand except for the replacement of thymine with uracil. See the figure below for an illustration of these ideas.