The version I'm most familiar with comes with the iPhone. There's a Find My Friends app. My oldest's iPhone is connected to my account with Apple, so if she (very foolishly!) decided to let me stop "following" her with Find My Friends, there's a different app called Find My iPhone that lets me find her phone, remotely lock it, cause it to emit a "I'm lost" sound even if it's muted, or remotely wipe it.

Android phones have similar functions. Some carriers have non-smartphone services to find phones (and their attached offspring) for a price.