You have been Selected by the Holy Grail as proven by the Command Seal that has appeared on you to be a master, summon a servant and fight in the Holy Grail War. 7 masters will be chosen under normal circumstances but it may differ based on how the war started.
There are five recorded forms of the Holy Grail War:
Grail War - 7 Masters each Summon 1 Servant for a Total of 7 and the last surviving
Master/Servant pair may make a wish to the Holy Grail.
Great Grail War - 14 Masters divided into two teams Summon 1 Servant per Master
and once only one team stands they can either agree on and make a wish or fight among each other in a normal Grail War. In addition a Ruler Class
Servant will be summoned by the Grail itself to oversee the war.
Grand Order - A single Master summons a Single Servant and fights through Singularity events, gaining at least one new allied Servant per completed Singularity.
Complete all singularities to save the timeline or fail and the world ends.
Moon Cell - 128 Masters each Summon a servant to fight in a weekly one on one fight in a tournament format controlled by the Moon Cell with the loser being erased.
Servant combat outside of these fights are banned but a dungeon is available to practice, gather resources and spy on other servants.
Subcategory - These wars have no set rules, each occasion is unique but tend to change the very nature and goal of the war from survival to hunts to puzzle labrynths.
Generally slightly safer then the others given the lack of rules results in less support from the Grail and reduces the usage of high cost abilities.
As a participant in the Holy Grail War, you will need to ready yourself, summon a Servant, and consider your mage-craft. Each of these elements will be required in order to survive the coming conflict and while there are other factors that will lead to victory or defeat, they are far smaller, uncontrollabled or must be addressed once the conflict begins.