It has been 14 years since Master Chief first stepped foot on Halo. Back in 2001 Bungie made a game called Halo Combat Evolved. This is a game that changed gaming forever.
In Halo Combat Evolved you played as a character named Master Chief, also known as John, was one of the world's greatest soldiers in the history of the human race. They were called Spartans.
  Now John came from the generation of Spartan 2's. The Spartan 2's started their training as children at the age of 6. This age was based off the Spartans we all know so well, back from the days of the Greek Empire and when there was Sparta. Spartans started their training at 6 years old because it was easier to train a child, being that their mind is so adaptive at that age. They found these kids from the best of the best. They took the smartest, fastest, and strongest kids of all the human race. Who started this project? Well it was Doctor Halsey. She started this project to create the strongest soldiers in the universe. They were made to fight the Insurrectionists. But as time moved on, they had a bigger role to play in saving mankind. 
  The main force of the military at the time was called the U.N.S.C Also known as the United Nations Space Command. But the Spartans were made under ONI, known as Office of Naval Intelligence, and the project start in the year 2517 and trained these kids until the year 2525. There were 150 kids that started the program. Each Spartan had a number assigned to them; the Master Chief's was 117.
As I have said, the Spartans were made to fight against the Insurrectionists, well that all changed one day when the Covenant attacked. The year 2525 the Covenant declared war on the Human race. They started with the Outer Colonies and worked their way to the center where earth was. After the Covenant had taken control of a planet they would glass I, killing billions of people. Glassing is what the Covenant did to get rid of the human remains. They used Plasma beams that came from their ship, and destroyed everything on the surface of the planets. The U.N.S.C then called upon the Cole protocol. In doing this a U.N.S.C. ship was attacked and defeated. Then the captain had to call upon the Cole Protocol, where they deleted all the data on the human race and where the locations of planets were. Then they were to do a slip space jump to a random location. They did this to help the human race live longer so they could have the time to try and defeat the Covenant. If the covenant had gotten the info on where earth was located, earth would have been destroyed as well as the human race.
The first Covenant ship to be destroyed, in a non-ship to ship battle, was destroyed by the Spartan 2's. They infiltrated the ship and deployed a massive bomb in the middle of the ship. It was then that Spartan 034, also known as Samuel, sacrificed himself to set off the bomb. In doing this, he let the other Spartans escape. Throughout this time Spartans were known to be invincible when Spartans started to die the U.N.S.C. knew that to keep morale up they could not let this happen. So they listed them as MIA, Missing in Action, instead of KIA, Killed in Action. With this the Spartans were known to live forever and to be known as the Earths greatest Super Soldiers. When the Spartans started to fight against the Covenant they started to call them Demons. This happened because they were so hard to kill, and the Spartans could take out 100's of Covenant troops.
The Covenant was split into different classes you can say. There is about 7 different aliens involved in the Covenant. It starts at the Grunts. The Grunts are small beings that are like the foot soldiers of their army. There are millions of Grunts and they traveled in groups to become more deadly. Next are the Jackals. Jackals are quick and agile. Most of them would carry around plasma shields, used to protect themselves. Some Jackals were also used as snipers. Next is the Hunters. The Hunters are basically big worms. They always travel in pairs.  Each Hunter has a giant plasma rocket attached to one arm, on the other they have a giant metal shield. Then you have the Elites. The Elites are kind of like generals. Each Elite travels with about 5-6 grunts and is there commander. If the Elite happens to die the Grunts will run with fear. Later on in the Human-Covenant war more aliens were discovered. The leaders of the Covenant were called the Prophets. There were 3 of them The Prophet of Regret, The Prophet of Mercy, and The Prophet of Truth. They were known as the Higharchs. They told of the Great Journey. In which the Covenant would became gods. They thought that in order to go on the Journey they had to cut through the Human, who were in their way. The Great Journey involved a giant ring that they called Halo. Which had the ability to destroy all living forms of life. So all Humans and aliens would die. They were made by a species called Forerunners. They made the rings to prevent the spreading of a certain alien.
It is called the Flood. The Flood would start as these little parasite things. They look like little brains with tentacles. These things would run around in hoards and attack a living being like humans or Covenant. They would bury themselves into someone chest and infect troops. In about a minute after that you will start to turn into the Flood. You would grow tentacles on your face and arms and will attack other humans. The Flood started their outbreak when the forerunners were alive. The Forerunners tried to fight them but they couldn’t. The Flood were spreading fast and the Forerunners were running low on people. So they decided to create Halo. This Halo would create a giant pulse that would kill everything that was living. There for stopping the Flood from spreading. They made 7 rings all together and spread them around the universe to cover the whole universe. But the only problem was that it killed all the Forerunners as well. In order to save the rest of the life forms they took samples of each species and saved them. After Halo went off and killed everything the Forerunners machines took all the specimens and put them back to their planets to live and prosper.  The Flood was thought to be destroyed forever. But was later found out that there was still some left.
Going back to the U.N.S.C they had their own ranking as well. It started at Marines. The Marines are the abundance of the Army, they have the most and are everywhere. Then they had the ODST's, also known as Orbital Drop Shock Troopers, they were the back bone of the UNSC. They could drop behind enemy lines and strike from behind. Then of course you have the Spartans.  
Each Army had their own vehicles as well. Starting with the UNSC you have the Warthog, Scorpion, Mongoose, and the Pelican are the main ones used in UNSC. The Warthog is a 3 seated jeep that has a gun turret on the back. The gun on back could be of 3 choices Machine gun, Goss, or Rocket. Next you have the Scorpion which is the tank. With a heavy turret as the main weapon and a machine gun turret next to it. Then you have The Mongoose which is basically like a 4 wheeler now a days and it fits 2 people. Then you have the Pelican which is the flying transporter, like a helicopter, of the Army. It can fit a whole squad in it at once. It is used to transport troops and vehicles around from place to place. The Covenant had they own vehicles as well. There is the Ghost, Wrath, Phantom, and the banshee.
Then you have the different weapons each Army has. The UNSC has the Assault Rifle, Battle Rifle, DMR, Magnum, Rocket launcher, and the Sniper Rifle.  The Covenant have the Plasma pistol, Needler, Plasma Rifle, the Energy Sword, and the Carbine are the main weapons they use. The Energy Sword was a special sword used by the Elites, it is a sword made of energy and can cut through people very easy. These are all the main weapons and vehicles that the UNSC and the Covenant use during the Human-Covenant war. 
During the Human-Covenant war the humans started to lose that is why they started to Cole protocol. Because of the Cole protocol Master Chief found Halo. If they didn’t find halo before the Covenant, like they did, the Covenant would have activated it and destroyed the universe. But the chief stopped them. Now this was just the beginning. All of this info lead up to the video game series that we know and love to this day.