Ransom-ware is one of the computer virus or malware that locks the computers files and demand money to return their files.
It is one of the growing threads for mobile as it can be hide in mobile apps. The mobile apps are one of the best loops holes for virus infection nowadays. Or it may be exchange through drive by download. To avoid threads user should be aware and careful about what they are downloading and which app they are installing and where the came form and check for trusted source and review of app store.
How does it work?
Like other computer virus it also may arrives to computer in form of spam or phishing email or fake software update and attempt the clicks or open as attachment. The virus start to work encrypting the users data.
Once the computer is locked down it demand the charge or fee to unlock. According to security it often demand bitcoins because it is not easy to trace and the value of bit coins are more now the value of bit coin is 1 bit coin =$500.
Sometime it just thread but generally virus really encrypt the files. There is no way to retrieve the file without paying the ransom but there is no guarantee the only a way to got data back is go to the backup copy.