Scareware is really similar to ransomware and will also try to extort you for money. Typically, they try the generate anxiety or panic by announcing that there are viruses or child pornography on your computer and that you must install software or pay a fee to remove the problem. They will typically display alarming warnings and other threats through pop-up windows.
The police will never notify you of a crime by email or on a webpage. Legitimate software providers will not harass you or send you threats to urge you to install programs. Any such warnings can be assumed to be scareware and should be ignored. You should activate a full scan of your system by your antivirus software if you see such scareware warnings.