Equestria

Equestria is the land in which My Little Pony: Friendship is Magic takes place. The population consists largely of intelligent, talking animals, ponies being the most prominent. Equestria is co-ruled by Princess Celestia and Princess Luna.

History
Long ago, Equestria was ruled by Celestia, who brought out the sun during day, and Luna, who brought out the moon at night. Luna eventually started to become bitter and envious since the ponies would sleep through her nights. Her growing jealousy transformed her into a shadowy figure named Nightmare Moon, and she was determined to make the night last forever. After her attempt to bring eternal night to the world, Celestia was forced to seal Luna away within the moon where she remained for a thousand years.

With the aid of the stars, Nightmare Moon eventually broke free, and tried to bring about eternal night once again. She was stopped this time by Celestia's student, Twilight Sparkle, and her five friends, who harnessed the Elements of Harmony to defeat her. Afterwards, Celestia offered Luna forgiveness, and Luna and Celestia now rule together once more. Since then, peace has generally been maintained, though various minor incidents have cropped up in and around Ponyville.

Appleloosa
Appleloosa is a Wild West town built by settlers about a year prior to the events of Over a Barrel. It is the home of Applejack's cousin Braeburn. The town is complete with the typical "wild west" buildings, such as a dance location, a saloon, and a sheriff. It was built on Buffalo lands without their permission, which caused territorial disputes. The name comes from the Appaloosa breed of horse.

Canterlot
Canterlot is home to the royal palace of Princess Celestia. It was also the home of Twilight Sparkle prior to her relocation to Ponyville. The name is a reference to Camelot, the home to King Arthur and his "Knights of The Round Table"

Cloudsdale
Cloudsdale is a city in the sky where the Best Young Flyer competition takes place and where Equestria's weather is made. Only pegasus ponies live and work there, since non-pegasus ponies cannot stand on clouds. It is also the hometown of both Rainbow Dash and Fluttershy. The name is a reference to the Clydesdale, a breed of draft horses.

Everfree Forest
The Everfree Forest is located outside of Ponyville. It appears not to be bound by the same laws as the rest of Equestria since plants grow, animals care for themselves, and the weather changes without the help of ponies. It is also home to some strange creatures, including Ursas and parasprites. Notable locations in the forest include: A comment by Applejack indicates that the forest is not actually part of Equestria.
 * Zecora's home
 * Froggy-Bottom Bog - A mucky swamp featuring thick bubbling mud, steep cliffs, and a hydra, among other dangers.
 * The Ursas' cave
 * The Ruins of the Ancient Pony Sisters - Where Twilight Sparkle and the others found the Elements of Harmony and defeated Nightmare Moon, turning her back into Princess Luna.

Fillydelphia
Fillydelphia is briefly mentioned by Celestia in Swarm of the Century where there is supposedly an infestation much like the Parasprite one that recently happened in Ponyville. The show's creator originally wanted to use the name for the main setting of the show but was asked to change it.

Hoofington
Hoofington is a village mentioned by Trixie as the site of her alleged battle with an ursa. The suffix -ington is derived from old English for "town". The name is quite similar to that of a political news website, The Huffington Post.

Manehattan
Manehattan is a large city where Applejack's relatives, the Orange couple, reside. Applejack is introduced to the high society of Manehattan in a flashback in The Cutie Mark Chronicles. She soon finds herself homesick, and returns to the farm. The name is a play on Manhattan Island, a section of New York City,

Ponyville
Ponyville is the main setting of the series. Ponyville was founded by Earth ponies although over time various forms of ponies have started living in the town.

Sweet Apple Acres
Sweet Apple Acres is a farm located near Ponyville. It is where Applejack and her family live and work.

Trottingham
Mentioned in Stare Master, Where Rarity is making 20 gold-lined gowns for ponies there. Seems to be a take on either Nottingham or Tottenham, a city in England and district of London respectively.

Whitetail Wood
A lovely forest near Ponyville. Unlike Everfree Forest, it is completely under pony control. Tree sap is harvested there.

Trivia

 * According to a post by Lauren Faust on DeviantArt, the Everfree Forest is west of Ponyville, Sweet Apple Acres is southwest, and Fluttershy's cottage and meadow are located between the orchard and the forest. Canterlot is northeast of Ponyville, and, based on the ending of the episode Swarm of the Century, Fillydelphia would be somewhere south of both of them.
 * In a letter to Princess Celestia, Twilight Sparkle refers to Equestria as a country. Technically, a country where the ruler's title is "princess" or "prince" is called a principality. Equestria was a diarchy - ruled by two persons equally - until the events one thousand years before the series, and was a monarchy during Luna's exile. Luna's current position is not made clear following the events of the pilot.
 * The English word most similar to the placename Equestria is equestrian, "of or relating to horse riding", from theLatin equester, "belonging to a horseman", from equus, "horse", which is also the origin of equine. In this sense, "Equestria" would translate as "land of the horsemen" instead of "land of horses". However, a more accurate toponym such as Equussia or Equinia would no longer rhyme with "Celestia".
 * Judging from the clock towers in Ponyville and Appleloosa, Equestria uses an 8-hour clock, which suggests a 16-hour day.