What is cipher ?

Hello ! So im new it the mgs cinematic universe and i have few questions. I played MGS3 , MGSV and im doing MGS2 right now.

In mgsv, the enemy is cipher, but it is a group or a code name for someone ? Like for zero or SF ? Or both ?

And I dont really understand zero and ocelot. In mgs3 zero seems to be friendly with big boss but in mgsv its an ennemi BUT in the truth tapes, zero talk about bb as a friend. And ocelot is more confusing to me. Is an ally to big boss, but work for cypher (?) and in mgs2 for solidus ?

My brain start to melt