I don't really know what a fork is, could anybody explain?
Bitcoin rely on their blockchain to keep their data.
The blockchain stays on many computers - everybody having bitcoin qt installed will download the whole blockchain (and most keep a local copy of it).
For whatever reason - intended (software change) or unintended (rogue miners, bugs, attack) the blockchain can split (fork).
This means that while blocks 1-x are identical, there will be 2 blockchains, one with blocks 1-x and then y1, and another with 1-x and then y2.
In most cases forks are short lived - you will not know it happened - and one chain is not used by anyone.
In some cases, like it was the birth of Bitcoin Cash, both forks (both versions of the blcokchain) exist, but they behave now almost like 2 separate blockchains.