Before I go into detail, I want to define some terms, so that I don't have to ramble all over the place with qualifiers and hemming and hawing.

• YOU are a kid. You don't have the cash to buy your own stuff, so most toys you get are gifts. Either they're wrapped up and given for special occasions, or bought for you in the store, the main point is that you get them because someone else is willing to pay for them.
• YOUR PARENTS are whatever adults you live with and who are responsible for you. They may not be your biological parents, they may include other adult relatives, but these are the people who have to live with the consequences of whatever gifts you get. So they have a vested interest in avoiding certain types of gift. On the other hand, they also know you pretty well, and are less likely to get you something really lame while thinking you'll like it.
• YOUR GRANDPARENTS are any adults who are allowed to give you gifts, but who do not live with you. Actual grandparents, aunts and uncles, friends of your parents, etc.

    Cool Grandparent Gifts are usually toys that make noise, shoot things, can damage the furniture or encourage violence...the more of these it does, the cooler it is. They range from the incredibly obvious (i.e. the remote controlled tank with realistic sounds that can fire foam balls and terrorize the cat) to the subtly insidious (i.e. by the 100^th time you hear Elmo do the Chicken Dance on Christmas Morning, you want to punt Elmo through a waaaaaalllll...). These are things you will love, but your parents will hate...but because it's a gift from someone, they're reluctant to take it away. At least, they won't take it away until you've driven them up the wall with it or set fire to the couch. Your parents would never buy one of these for you, or let you use your allowance to buy it. But because it's a gift from your grandparents, they let you have it at least long enough to make them regret doing so.

    Motivations for giving cool grandparent gifts can vary. On the innocent end of the spectrum, the grandparents may simply not have thought through the implications of the gift, being focused more on giving you something you'll really like. This is common with the insidious ones, that seem harmless enough unless your grandparents realize that you will probably hit the sound and light gimmick on the toy every ten seconds for an hour. Often, though, there's a far less innocent motive:

    "Look, Timmy loves his new drum set! Maybe now you won't have to worry about him going at your best pots and pans with a hammer like I remember a certain little boy doing once...." Literal grandparents often give cool grandparent gifts specifically to get decades-removed payback from their own children (your parents), perhaps because your parents got cool grandparent gifts as kids too. So when they smile as they give you the nicely-wrapped bundle of chaos, at least part of that smile is directed at your parents, with an implied, "I told you that you'd know what it was like when you had kids of your own!"

    Of course, it need not be direct payback, especially if the "grandparents" are not literally grandparents. But these are gifts that are meant to keep on giving...your parents a headache. And you can expect that if the gift-givers have kids your age, your parents will be paying back the favor when it comes time to give gifts to those kids.

    Lame Grandparent Gifts, on the other hand, almost always come from innocent motives. Now, I'm not talking about scarves and socks and sweaters here, at least not in general. Those are just lame gifts, period. But lame grandparent gifts happen when your grandparents think they're getting you something you'll really like, but they aren't. Specifically, I'm talking about those aisles full of branded crap that show up in stores around the holidays...the racks of watches with cartoon characters printed on the faces, the pen and pencil sets, the shampoo bottles, the cup/plate/bowl sets.

    Now, granted, under some circumstances, these things can be cherished. It's not uncommon to find a kid who refuses to drink from anything but his Batman glass. Or for parents of a girl to have to surreptitiously refill that Disney Princess shampoo bottle with shampoo from another source because the girl won't use any other brand (and that particular bottle is no longer sold). While they tend towards lameness, they can still be cool gifts if the giver knows you well enough. But since by the definition above, grandparents don't live with you, odds are they don't know you well enough.

    What makes them lame grandparent gifts is a combination of two factors:

1. Your grandparents guessed wrong about what property you like.
2. This was given in place of a real toy.

    Of course, your grandparents can't be blamed too much for lame grandparent gifts. As I mentioned earlier, aisles full of these things will crop up around the holidays, to take advantage of grandparents who don't normally buy toys and have little to no idea what you'd like. So all of these kid-popular brands arrayed with semi-sensible gifts are just a trap laid for your grandparents by wily stores who have healthy profit margins on a $1 watch turned into a $6 watch by slapping a Hannah Montana sticker on the face. Of course, your grandparents are probably only buying these things because they're being careful to avoid any cool grandparents gifts (not wanting to be mean to your parents), so you're still a little justified in resenting them. And if they knew you a little better, they could have gotten you something non-lame that wouldn't annoy your parents...or at least known that you'd wear a Transformers watch, but not a Batman watch.