Everything you need to know about Apple's heated battle with the FBI over the locked San Bernardino iPhone

The battle between the FBI and Apple over a phone used by one of the San Bernardino shooters is poised to be one of the biggest technology stories for years.

In short, the FBI wants Apple to help it hack a phone it can't access because it has its passcode lock turned on.

The FBI says that Apple is obstructing a critical investigation with national security implications by refusing to build custom software to bypass the phone's passcode. 

Apple says it's unfair that the government can tell companies they have to do something that they don't necessarily want to, and that what the FBI wants endangers all of its customers.

In short, there are big differences in outlook between the two groups, and both have the budget and desire to fight this to the end.

Here's what you need to know about how it all started, and where Apple's battle with the FBI stands today.