DNA contains coded instructions that your cells need to operate. In a eukaryote, an organism with a nucleus in each of its cells, the DNA is stored inside the nucleus, so those instructions have to be transmitted to the cell by first making a copy of them in a polymer called messenger RNA or mRNA. mRNA is edited by cellular machinery before it leaves the nucleus, and several important molecular features are added to it to mark it as finished and ready for use.
The first chemical modification which all eukaryotic mRNAs share is called a 5' cap. The RNA polymerase enzyme travels along a strand of DNA making an RNA copy or transcript. The end of the RNA polymer where the RNA polymerase started synthesizing is called the 5' end. Three other enzymes add a chemical group called 7-methylguanylate to the 5' end; this modification is called a cap. If an mRNA appears in the cell without a 5' cap, it may get broken down by other enzymes; the instructions it contains will never be translated. The 5' cap marks the mRNA as legitimate and protects it from degradation.
The other universal modification found only in eukaryotic mRNA is a poly-A tail. The 5' end of the mRNA is where the RNA polymerase started, and the 3' tail is where it ends. Following transcription, an enzyme called poly(A) polymerase adds anywhere from 100 to 250 additional adenosine or A subunits, hence the name poly A tail. This tail appears to make the mRNA more stable and marks it as destined for export from the nucleus.
Functions for Modifications
5' caps and poly-A tails are found in all eukaryotic mRNAs. However, bacteria and other prokaryotes also use mRNA, but their mRNAs lack these two characteristics. Eukaryotic mRNA is sometimes edited or spliced before it leaves the nucleus, so they need to regulate which mRNAs can leave the nucleus. Moreover, translation of the instructions encoded in the mRNA is a much more highly regulated process in eukaryotes, and these modifications also play important roles in that process. Unlike eukaryotes, prokaryotes have no nucleus so there's no need to regulate entry or exit of mRNAs -- as soon as the mRNA is transcribed it's set loose in the cell.
Viruses and mRNA
When a virus infects a eukaryotic cell, the pathogen needs to ensure that the host cell stops producing its own proteins and starts making viral proteins and RNAs instead. Some of them like polioviruses and picornaviruses carry an enzyme that chops up a protein required to translate the instructions stored in a 5'-capped mRNA. As a result none of the cell's own mRNAs are translated, and the viral RNA that is not capped is translated instead. By so doing they take what could be a liability -- their own lack of a 5'-cap -- and turn it into an advantage.