If you've never had to deal with “ransomware”, you're in luck. A particularly malicious type of software, ransomware puts your personal data at risk by requiring you to pay certain people to get it off your machine or face file deletion. While originally quite crude, ransomware is becoming more and more sophisticated, now commonly employing encryption that is tough to break. While an adequately equipped lab may be able to deal with the current generation of encryption, in the future that may not be the case:

"We were able to decrypt 330 and 660-bit keys within a reasonably short space of time, but a new variant, with a longer key, could appear at any time. If RSA, or any other similar algorithm which uses a public key, were to be used in a new virus, anti-virus companies might find themselves powerless, even if maximum computing power was applied to decrypting the key," warns Aleks Gostev, senior virus analyst, Kaspersky Lab.
While this is yet again another very strong reason to backup important data often, that's no solace to those who have become victim to those that propagate this type of software.