DNA or deoxyribonucleic acid is double-stranded and contains thymine, whereas mRNA (or messenger ribonucleic acid) is single-stranded and contains uracil. The sugar molecule in DNA is a pentose sugar (deoxyribose, i.e. one less oxygen), whereas mRNA contains ribose (a pentose sugar molecule with 1 more oxygen than deoxyribose). DNA contains equal amounts of purine and pyrimidine bases, whereas mRNA may not.
The main function of DNA is to store and transmit genetic information necessary for existence and survival of an organism. The main function of mRNA is to carry genetic information from chromosomal DNA to the cytoplasm for protein synthesis. DNA has a long life span (although it may evolve over that time period resulting in evolution), whereas mRNA withers away after few translations.