Transcription is a process where a copy of mRNA is formed from one of the DNA strand. For this, DNA strand has to open up and complimentary bases are added to make the mRNA copy. But this copy doesn't include the portion which is not part of the genes. This waste portion is called as "intron".
Also in RNA's, thymine is absent and instead uracil is present.
DNA - TACCGT
mRNA- AUGGCA
A pairs with U; C pairs with G and T pairs with A.