Viruses are difficult to classify because while they have several characteristics of living things, they do not have several others. They currently are not considered to be living organisms, although this is being reconsidered.
Viruses are very simple organisms, usually consisting of a protein coat and either DNA or RNA. Having genetic material and the ability to produce more of themselves would seem to indicate a living thing. However, they are not involved in transforming energy (they do not perform cellular respiration or photosynthesis), and they are not capable of reproduction on their own. Viruses take over and reprogram cells of living organisms so that the host cells do not make copies of themselves--they instead are reprogrammed to make more viruses.
Because they are not living, viruses are difficult to classify. The classification of living things generally involves assigning organisms to a specific place in one kingdom or another and giving the organism a genus/species name. Viruses instead are usually classified on what they look like, whether they contain DNA or RNA, or what organism they infect.