A virus is a non-living obligate intracellular parasite. They can only replicate inside of a living cell as they lack the necessary enzymes and molecular building blocks to be self sufficient. Viruses can be classified by structure (icosahedral, enveloped, complex etc), genome (retroviruses have an RNA genome) or by the route through transcription (Baltimore classification).
They have a large protein coat, called a capsid, which is formed from many repeating protein subunits which join non-covalently to produce a large icosahedron sphere. Inside this sphere is where the virus' free nucleic acids are kept. The structure of this sphere is such that is protects the nucleic acid but also allows the nucleic acid to exit so it can go on to infect other cells.