A virus is any program that reproduces itself by using the resources of your computer without your knowledge or consent. A virus just copies itself and spreads. A virus cannot move by itself, it must attach itself to another program. Viruses may be written to multiply, to damage other programs, or to alter data. There are many different kinds of viruses.
Viruses come in a wide variety.
They are often small and well hidden, therefore difficult to find. Some are slow while others are fast. Some take days, weeks, months or years before they start damaging a computer. They may be benign and result only in amusement or mere annoyance, or they can be malignant and malicious. There is something wrong with your computer, we just can't figure out what.