The cameo is one of television's oldest tricks to drum-up interest for a show. But not all cameos are created equal. Inside, a list of the kinds of cameos, why they exist, and just how effective they are.

Mixing and matching these cameos is definitely allowed, as they are not mutually exclusive from one another.

The Inside Joke

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Those two people are dating in real life, but they play enemies on the show! Hopefully you know about their personal lives and pasts, because if not, you're not gonna get it.

Example: Brad Pitt on Friends while he was dating Jennifer Aniston.

Entertainment Value: 4. Even when you get the joke, it's not that great.

The Awkward Athlete

If an athlete wins a championship or a gold medal, they will definitely attempt to cash-in on their 15 minutes of fame. Only one thing, they can never ever act. These are always cringe-inducing.

Example: Keith Hernandez on Seinfeld, Kerri Strug on Beverly Hills 90210

Entertainment Value: 7. It's such a fish-out-of-water situation one can't help but but tune-in to watch a basketball player nervously flub their lines.

The Lesbian Kiss

It's always someone famous. There will never be a lesbian kiss on television that features a no-name actress. This is usually just boner fuel to appeal to the boys who remember those two girls making out in Cabo during spring break. Although Muriel Hemingway making-out Roseanne wasn't nearly as boner inducing.

Example: Jennifer Aniston and Winona Ryder on Friends. Jennifer Aniston and Courtney Cox on Dirt.

Entertainment Value: Depends. Very high if you're a male between the ages of 13 and 20. Very low to everyone else.

The Crossover

Two shows run on the same network, one is popular, the other isn't. So the network makes a character from the popular show make an appearance on the unpopular one to create interest in the floundering show.

Example: The Critic and The Simpsons, Friends and The Single Guy, Urkel and every show on ABC during the '90s.

Entertainment Value: 5. It's too much of a ploy to drum up interest for a struggling show.

The Dramatic Actor Turned Comedian

Juxtaposing an actor that usually plays tortured, involved, difficult roles in a comedic situation usually gets terrific results.

Example: Charlize Theron on Arrested Development. Almost every episode of Ricky Gervais' Extras.

Entertainment Value: 9. Great actors are great actors whether it's comedy or drama.

The Comedic Actor Turned Dramatic

When an actor plays harmless, funny roles all of the sudden, plays a homicidal pedophile on TV, it really adds another level of creepiness.

Example: Darrell Hammond on Damages, Martin Short on Law and Order: SVU.

Entertainment Value: 9. It's like children in horror movies: always effective.

Getting Animated

It's always more admirable when an actor lends their voice to play somebody other than themselves. Michelle Obama did this last night on The Simpsons.

Example: Dustin Hoffman on The Simpsons as Mr. Bergstrom, Lisa's substitute teacher, George Clooney as a gay dog on South Park, Elizabeth Taylor as the voice of Maggie Simpson, David Cross as Bert Banana in Aqua Teen Hunger Force.

Entertainment Value: Depends. Animated cameos are so common nowadays, they encompass almost every type of cameo on this list. Usually the more random the better, like Elizabeth Taylor being the voice of Maggie.

The Unexplained Cameo

An famous actor shows up on screen for a brief period of time with little to no explanation. Well, other than it will get people talking (if they caught it).

Example: Ben Affleck's completely unexplained appearance from last season's Curb Your Enthusiasm.

Entertainment Value: 5. Not terribly entertaining, as it's a blink and miss it appearance. but if you do happen to catch it, it can haunt you for days.

The Self-Effacing Cameo

An actor with a certain reputation goes on a TV show to show that they don't really have a stick up their ass! They adore comedic things! They will always play themselves, and they will always be terrible at doing so. And show us that they usually don't have a great sense of humor.

Example: Yoko Ono on Mad About You, Al Gore on 30 Rock.

Entertainment Value: 7. Similar to the athlete cameo. Total fish-out-of-water situation.

The Blatant Ratings Grab

A huge celebrity with no discernible skills in acting wants to be on TV. They are so famous, they will no doubt bring in big numbers. Of course some terrible sitcom is more than happy to oblige.

Example: Britney Spears on Will and Grace and How I Met Your Mother. The last three seasons of Friends.

Entertainment Value: 2. It's more annoying than anything else.

Thanks to commenter DahlELama for inspiring the idea.