Well, I have learned a lot from Wikipedia.
It tells me that：
A computer virus is a malware program that,
when executed, replicates by inserting copies of itself (possibly modified)
into other computer programs, data files, or the boot sector of the hard drive;
when this replication succeeds, the affected areas are then said to be
"infected". Viruses often perform some type of harmful activity on
infected hosts, such as stealing hard disk space or CPU time, accessing private
information, corrupting data, displaying political or humorous messages on the
user's screen, spamming their contacts, or logging their keystrokes. However,
not all viruses carry a destructive payload or attempt to hide themselves—the
defining characteristic of viruses is that they are self-replicating computer
programs which install themselves without user consent.
Virus writers use social engineering and
exploit detailed knowledge of security vulnerabilities to gain access to their
hosts' computing resources. The vast majority of viruses target systems running
Microsoft Windows, employing a variety of mechanisms to infect new hosts, and
often using complex anti-detection/stealth strategies to evade antivirus
software. Motives for creating viruses can include seeking profit, desire to
send a political message, personal amusement, to demonstrate that a
vulnerability exists in software, for sabotage and denial of service, or simply
because they wish to explore artificial life and evolutionary algorithms
