Ransomware is malware that will pop-up a message telling you that your computer is infected but just pay some dollar amount and this application will remove it for you.
In the past most of this ransomware was installed by malicious software installers when you installed something you thought was desirable on the web.
In the past these have been exclusively targeted at Windows machines because Windows is such an easy target, especially if the user was using Internet Explorer with active-X.
More recently though browser based versions have emerged that affect MacOS as well and even, Gasp!, Linux. However these newer versions often reside in the browser cache and just clearing the cache usually gets rid of them. Sometimes they also will add advertisement bars or other trash that needs to be removed, and often the removal function is disabled. The work-around for this is to reset the browser to the default configuration and then re-install all the plugins that you normally use.
If you have a Windows machine that gets infected with the more traditional versions, MalwareBytes, the free version, almost always will find and remove them, and paying the requested ransom most certainly will not.
If your computer gets infected and you need assistance cleaning it up and you are in the Puget Sound region, I will come to your location and clean and optimize your computer at a cost of $25/hour including travel time. I can also assist you with configuring peripherals, installing and configuring operating systems and applications, and general optimization for this price. If you are interested, please e-mail firstname.lastname@example.org.