If you are looking for a literal answer, then Assassin's Creed is an open-world historical action-adventure RPG game with hints of stealth-action play and whose focus is the deadly arts of the Assassin. With the release of AC3 in late 2012, the game basically consists of five main games and a significant number of supporting titles (seven in total) that represent the extension of the story and some of the characters, but it really blurs the lines at this point too, because in addition to its visceral game presence it also appears as Comics, Films, Novels, and the distinct uniform of the characters have crossed over into the popular culture, to the extent that the costume has appeared in a diverse collection of other games that have nothing to do with the series!
The very American Logo Screen for AC3 leaves little doubt of the setting
I know, you are wondering what games those are, right? It is not just a few either mind you - try Academy of Champions: Soccer (Wii), in which the character Altair is offered as a playable character (along with some other Ubisoft characters), and in 2012 in the game Soulcalibur V, Ezio appears as a playable fighter. In the 2008 title Prince of Persia, Altair's Costume was offered as an unlockable item if you had the pre-order code for it, and the series even appears in the PS3 classic LittleBigPlanet, as the main character Sackboy can be costumed as Ezio!
In Prince of Persia: The Forgotten Sands there is an unlockable outfit that is available via the Uplay Network as a reward, and the costumes even appear in the Final Fantasy series, in FFXIII-2, which has a costume of Ezio from Assassin's Creed: Revelations as DLC. There are other examples, but you get the idea; the cultural impact that the game series and its characters have had have turned the Assassin of Assassin's Creed into an icon!
At various points the games have appeared on Windows, PS3, Xbox 360, Mac OS X, as well as on the Nintendo DS and PlayStation Portable, PlayStation Vita, and iOS. There are mini-game versions for Android, Nokia Symbian and Windows Phone platforms, and of course on the Wii U, but interestingly enough no single platform has ALL of the games on it!
The Premise of the Assassin's Creed Saga and Series
If you have been playing the games from the beginning you already have a lot of time and interest invested here, so you likely know all about the backstory, but if you have not, then understanding the premise of the game and series is crucial to properly appreciating what you are doing and why you are doing it!
The events in the Assassin's Creed saga take place in the year 2012, and largely focus upon the story protagonist, Desmond Miles, who is a bartender and the descendant of several historic lines of prominent Assassins who are part of the organization going back hundreds of years. Desmond was supposed to be raised as an Assassin, having grown up in a volatile and highly mobile family, moving from safe house to safe house, but that nomadic lifestyle was anathema to him, and he ends up running away to find what he hopes will be a more fulfilling existence, eventually becoming a bartender in a hip club in the big city. And that is really where all the trouble starts.
Kidnapped by what at first appears to be a megacorporation called Abstergo Industries, this shadow entity turns out to be the modern-day face of the Knights Templar - the sworn enemy and foe of the Assassins and the threat that they have been battling since their founding! The Templar are very much aware of just who Desmond is and what his ancestral lineage is, and that is really the point - it is that lineage that they mean to exploit.
There is a very real theory that each human is created with the stored genetic memory of all of the people that proceeded them in their genetic maternal line - this theory has been said to explain the phenomenon of past lives memories, savant talents, and even natural artistic abilities, the concept being that we only use a small portion of our brains, and the rest are untapped but populated memories. The Templar have created a machine called "The Animus" that allows us to tap into those memories and relive them in the artificial reality construct that it creates by scanning and reading them, and then recreating their environments.
The story of Desmond Miles and the Templar so far
Desmond was kidnapped for the genetic memory he holds, and so is forced to use the Animus to experience his those ancestral memories in order to assist the eggheads at Abstergo in finding the location of artifacts that they call "Pieces of Eden", artifacts that hold immense power, and allow those who possess them to not only control mankind and alter its fate, but bend the fabric of reality itself! Fortunately for Desmond in the midst of this quest he is saved by a small team of modern-day Assassins who infiltrated Abstergo and mean to prevent the Templar from succeeding at their goal.
Agreeing to work with them, Desmond uses a version of the Animus that the Assassins have built on their own - called the Animus 2.0 & 3.0 - to continue mining his genetic memories in order to discover the locations of additional Pieces of Eden so that they can be recovered before Abstergo can get to them. In the process of experiencing these memories some of the wicked abilities of these Assassin relatives genetically leak into the real world, and Desmond starts to acquire skills from the legendary Assasssins in his family - this effect is known as the Bleeding Effect - and there may well be a downside to it, mainly the potential for insanity due to living with multiple sets of memories and personalities in his head!
Inside the Animus Desmond relives the adventures of Altair ibn-La'Ahad, a Syrian Assassin who is seeking to redeem himself for mistakes that he made that turned into a dark mark in his life; the Italian Playboy Assassin Ezio Auditore da Firenze, and the part Native American Ratonhnhaketon (who is also called Connor), the role we play in this game.
Connor is seeking more and less than you might expect - bearing in mind that he is not aware of Desmond as such, his goal is really to protect his tribe and its way of life, and considering that the time line he lives in is the American Revolution - an era in which Native Americans were often simply executed on sight, that is some tall order!
As the events unfold Desmond learns of the prophetic "End of the World in 2012" that has been predicted by the Mayans, the information coming from the former Animus test subject named Subject 16 who, in past adventures, has both hurt and helped him. He also picks up clues with respect to the prophecy from the holographic figures of a species that pre-dates humanity and that died out during a massive natural disaster that occurred long before humanity spread through the world. They were killed by a massive series of solar flares that nearly wiped out all life on earth, and it seems that this prophecy of doom really relates to a second similar and approaching event!
So you see the story is about Desmond, the Animus, the Pieces of Eden, and the Prophecy - and the Pieces of Eden may be the only way that he can acquire their ancient knowledge, discover the truth behind the prophecy, and with a little luck, find the Pieces of Eden he needs to save the world... No pressure, no stress, you can do it!
Well there you have it, so relax, be prepared to suspend disbelief, and sink your teeth into this continuing adventure as you work towards the goal of saving the world.
A logical question to ask yourself - if you have not played the games in this series, or even if you have - is just exactly what is this Assassin's Creed thing? It is an odd name that begs the question, and the simple answer that it is a lengthy and complicated story about the quest of a ragtag band of resistance fighters who, while they have their origins in the Middle East, eventually grew and expanded to throughout the world, so that in the modern era even the idea of defining them as the ancient order of Assassins that they so clearly are born of is absurd. So just what is Assassin's Creed then?