What is a virus?
A computer virus is a type of malware 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". There are many types of viruses for ex. trojan, trojan horse, worms, backdoors, stealing and other. The only way to protect yourself from them is to download or buy antivirus.
How viruses get in our computers?
They can be fake files like a game or facebook hacker which when you run it (if you haven't got antivirus of course) it will get into your sistem and it can delete some data, steal the credit card password, restart your computer every time when you start it and many other things. So be careful when downloading something!
How can you delete them?
As i said you can delete them with antivirus softwares like Eset, Avast, BItDefender, AVG and many others.
When your AV detects something and you don't think that your program is a virus then it's called a false positive, for ex. you want to download a hack for Counter Strike and if your antivirus detects it it can be a false positive.