You can blame the FSF for "Restricted Boot" as a term. It's not a term Microsoft or the UEFI Forum uses, and they use "Secure Boot" to cover both versions of this.
But, if you want to explain it to your grandmother, imagine that the auto industry had widespread problems with gas stations selling watered-down gasoline. You're driving along the highway, you refuel at the closest gas station, and two miles down our engine starts sputtering.
Ford says, "We've got a solution: we're making our cars with special patented gas tanks, that only fit our special patented gas nozzles. You can only fill up from Ford-run gas stations, and unapproved gas stations can't make nozzles that fit your car any more." While this does solve the problem as stated, it's also pretty anticompetitive. This is Restricted Boot -- you buy a computer, and you can only install operating systems that the computer vendor approves.
Customers complain, and Ford says "Okay, fine, you don't want us in control of where you get gas. Fine. We'll let you swap out the special gas tank cap. If you decide you like Chevron gas, go buy a Chevron-patented gas tank cap from them -- but we take no responsibility for whether Chevron is doing a good job of franchising their gas stations to trustworthy people. If you want to live dangerously, we'll even let you unscrew the connector and let _any_ nozzle fill fuel, but if your engine burns out, don't complain to us." That's Secure Boot. By default, the computer vendor restricts boot to OSes that they've vetted, but you as the computer owner can choose to change who's trusted to write OSes, or even trust anyone at all.
Chrome OS's solution (which Matthew has blogged about before) is like this story, except that there's no option for third-party fuel vendors to provide replacement special caps. You either have to trust Google, or trust everyone. It's a lot better than Restricted Boot, where you can't remove the special cap at all, but it's not quite as good as Secure Boot, since you have no protection if you opt out of trusting Google.
The distinction is that boot verification uses cryptography instead of patents, so it actually works. :)