So you're convinced that the Mass Effect universe is a science fiction epic for the ages but you don't know if you have the time to explore it. All told you can expect to spend 100 hours or so playing through all three Mass Effect games. That's about the time commitment of watching every single episode of a show that's been on the air for seven seasons. Maybe you're looking to cut a few corners and skip a game or two.
For those of you looking to jump right in at the third game, here's a brief overview what you need to understand in order to play ME3. Mass Effect 1 and 2 are both great stories and I'm trying to hit the highlights without ruining the games entirely if you decide to go back and play them later, but even these basic story terms will ruin some of the twists from the original Mass Effect.
Reapers
The Reapers live beyond the edge of the galaxy. As their name implies they return every 50,000 years or so to kill off all the advanced races in the galaxy for reasons that we don't yet understand. They've performed this cycle countless times and they've returned to kill of humans and the other races of the Mass Effect universe.
They're Like: The Borg crossed with the Elder Gods
The Citadel
The Citadel is a million year old space station of immense size that's used as a kind of space U.N. by the different races of the Mass Effect universe. The Citadel was found by the Asari when they started exploring space but it was actually constructed by the Reapers as a kind of galactic trap to make it easier to find and exterminate advanced life when they return.
It's Like: Babylon 5. It's kind of impossible to ignore the parallels.
Turians
Turians are a militaristc heirarchical race that Humanity went to war with when we first started exploring the galaxy. They join the Salarians, the Asari, and Humanity as the 4th of the Council races.
They're Like: If the Cardasians weren't such dicks.
The Council
All of the space-faring races in the galaxy have a place on the citadel but the most powerful races sit on The Council which is essentially the intergalactic governing body of the Mass Effect universe. There are four races on The Council including humanity which was inducted at the end of Mass Effect 1.
They're Like: The U.N. Security Council in Space.
Spectres
Spectres are the elite special operatives of the Galaxy. They can go anywhere and do anything to investigate anything that concerns them. Spectres are above the law and answer to no-one in the galaxy except for the Council. Commander Shepard became the first human Spectre during the events of Mass Effect 1.
They're Like: Being Space Jack Bauer.
Asari
The Asari are an "asexual" race that conveniently consists entirely of attractive blue women. They're the first space-faring race from this cycle and they were the first race to reach the Citadel. As a result they have some of the most advanced technology the galaxy has to offer.
They're Like: Twileks and the Minbari had babies.
Salarians
The Salarians are a short-lived but brilliant amphibian race. As one of the four races on the Council they're a major power in the galaxy. Since they're relatively physically weak they prefer to out-think their opponents. This is why they uplifted the Krogan to fight the Rachni and why they used the Genophage to contain the Krogan when they got out of hand.
They're Like: Supergenius frogs.
Mass Relays
Although ships in the Mass Effect universe are capable of some minor FTL flight it would still take hundreds of years to cross the galaxy at their speeds. The Mass Relays are the only way to really get across the galaxy. The Mass Relays were constructed by the Reapers to make sure that civilizations evolved in the direction they choose and none of the races in the Mass Effect universe know quite how they work.
They're Like: Stargates for spaceships.
Krogan
The Krogan are a battle-hearty race of reptilian aliens that have multiple redundant organ systems and the ability to birth new offspring in clutches of hundreds at a time. The Krogan were uplifted into a space-faring race by the Salarians to help fight a war against the Rachni, Mass Effect's obligitory hive-mind race of giant insects. After the Krogan started threatening to take over the galaxy the Salarians unleashed the genophage, a genetic disease that attacked the Krogan and prevented them from producing viable offspring. This, understandbly did not sit well with the Krogan.
They're Like: Bigger, angrier Narn.
Protheans
The Protheans ruled the Galaxy before being wiped out by the Reapers 50,000 years ago. During their time as the most advanced species in the galaxy they monitored and guided humanity and most of the other species in the Mass Effect universe.
They're Like: The Ancients from Stargate with a much sadder backstory.
Quarians
The Quarians were driven from their homeworld by an A.I. race they constructed called the Geth. The remains of Quarian society now travel the galaxy in a migrant fleet that gathers resources in the hopes of one day taking back their homeworld.
They're Like: The Humans of Battlestar Galactica except they're stuck in environmental suits all the time.
Geth
The Geth are the A.I. race that drove the Quarians from their homes. Although they're primarily antagonists in the first game, Mass Effect 2 had several developments, including a Geth crew member, that implied they were a mostly peaceful race with as much right to exist as the Quarians.
They're Like: Cylons but without Tricia Helfer.
Biotics
Biotics can manipulate matter with their mind with the help of some special equipment. There are a number of ways biotics can use their powers for combat use but they all essentially run on manipulating the mass of either the biotic themselves or their opponents. Human biotics are a relatively recent development caused by an accident but a number of alien species developed biotic powers naturally.
They're Like: Telekenetics but with SLIGHTLY more science behind it.
Element Zero
Element Zero or eezo, is the substance responsible for manipulating Mass in the Mass Effect universe. All FTL ships and Biotics use Element Zero or Eezo to manipulate matter. With some eezo and a little electricity you can raise or lower the mass of an object as you see fit which makes for FTL travel and cool telekentic powers.
It's Like: Unobtanium or any number of other sci-fi substances. It's the magic sci-fi element that makes the space magic run.