A virus is a micro-organism that is designed to infect its host. The primary purpose of this micro-organism Is to deliver its DNA / RNA into a host cell. Viruses are parasites (meaning they need another organism to survive) and are generally smaller than bacteria.
You can see bacteria under a normal microscope and need an electron microscope to be able to see viruses. Viruses are measured in nanometers (nm) and bacteria is micrometres (um). There are 1000 um in 1 millimetre (mm) and 1 million nm in 1 mm.
The most common viruses we hear about daily are HIV, Influenza & Ebola. As mentioned, the primary focus of a virus to replicate its DNA / RNA and can infect a variety of hosts in a few ways.