Viruses are the simplest and tiniest of microbes; they can be as much as 10,000 times smaller than bacteria. Viruses consist of a small collection of genetic material (DNA or RNA) encased in a protective protein coat called a capsid. (Retroviruses are among the infectious particles that use RNA as their hereditary material. Probably the most famous retrovirus is human immunodeficiency virus, the cause of AIDS.) In some viruses, the capsid is covered by a viral envelope made of proteins, lipids and carbohydrates. The envelopes may be studded by spikes made of carbohydrates and proteins that help the virus particles attach to host cells. Outside of a host, viruses are inert, just mere microbial particles drifting aimlessly.
There are thousands of different viruses that come in a variety of shapes. Many are polyhedral <polly-hee-drul>, or multi-sided. If you've ever looked closely at a cut gem, like the diamond in an engagement ring, you've seen an example of a polyhedral shape. (Unlike the diamond in a ring, however, a virus does not taper to a point, but is shaped similarly all around.) Other viruses are shaped like spiky ovals or bricks with rounded corners.
Some are like skinny sticks while others look like bits of looped string. Some are more complex and shaped like little lunar landing pods.