Mana Accelerators are special spells, normally permanents, that allow the player to gain more mana to play other spells. Some of the best-known mana accelerators are among the "Power Nine", namely the "Moxes" and the infamous "Black Lotus". Both the Moxes and the Black Lotus have inspired many other card designs like, for example, the Chrome Mox or the Gilded Lotus, all cards that can be categorized by the term of Mana Accelerators.
Mana Accelerators don't need to be cost effective in the turn they are played, even though cards like that exist (every Mox, a lot of Lotus-themed cards, Sol Ring). Mana Accelerators are mostly used in the early turns of the game to boost the mana production allowing the player to get an edge over the opponent.
While normal lands certainly give mana to allow playing costlier spells, one wouldn't count them as Mana Accelerators. That, however, doesn't mean that lands can't be Mana Accelerators too; examples for cards like this are Ancient Tomb or the Locus cards from the plane of Mirrodin.
Artifacts are normally the way to go when thinking about Mana Acceleration. While artifacts aren't always better than their colored counterparts, they can be picked up by every colored deck without worrying about paying with certain mana. Therefore, including some artifacts that boost the mana gain can be a reasonable choice.
Colorwise, most of the Mana Accelerators are green spells, since the normal strategy of the color green is to ramp up a lot of mana to bring more and/or bigger creatures to the battlefield, either rushing the enemy with a lot of tokens and playing one buff spell to make them powerful or to overwhelm the enemy with gigantic monsters. Green mana accelerators are mostly enchantments and their effects will often increase the mana that lands are producing, but also include a good amount of creatures who are normally able to be tapped for mana. The increase of mana production per land can be amplified even further when combined with one of many green cards that allow the player to search his/her deck for lands or even allowing to play more than one land per turn.
The color red has its own way of getting mana fast, if you wish to play a deck that requires either a hefty amount of mana or playing with a combo or a storm counter in mind. It uses spells that will increase your mana by a good amount, but only for the time when you play those spells, giving you a fast and instantaneous boost instead of a permanent one. Examples for Sorceries that work this way are Rite of Flame, Seething Song or Desperate Ritual.
The last color that might be noteworthy in producing mana is black, since it has a small amount of spells that work similar to examples given above. For a more permanent mana boost, the Cabal Coffers or the eponymously named Magus of the Coffers can help providing an insane amount of mana in later stages of the game. For a more short-term approach to the mana building game, one can rely on Black Ritual, Cabal Ritual, Sacrifice or Burnt Offering.
While blue and white have a small amount of cards that allow to boost the mana pool without relying on artifacts, those cards are seldom useful enough to be played. If really necessary, one can choose to be play cards like High Tides to boost the mana production of the deck.