An Interchangeable Core is a lock core that can be changed without fully disassembling the lock. The cores generally come in a "Figure Eight" form factor. Interchangeable cores are generally also characterized by their ability to be changed by the end user, rather than a locksmith.
The opening and closing of an interchangeable core is exactly the same as it would be on a full form factor lock of the same type. (For example Pin tumbler.)
In most interchangeable cores changing of cores is done by using a special key known as a "Control Key" or "Interchange Key". This key functions as a normal key would, but it unlocks the holding mechanism for the core, allowing it to slide out.
Other interchangeable cores (mostly on padlocks) use a single retaining screw below the clasp to hold the core in place, however there is debate about if this is still technically and interchangeable core.
Interchangeable cores have both security benefits and downsides.
The addition of an extra one or more pins controlled by the control key leave at least one extra possible correct position for the lock. Making it easier to pick.
However the time saving benefits of an interchangeable core and the fact the work does not need to be completed by a locksmith mean that in the event of a key breach, locks can be changed quickly and efficiently by staff rather than a professional locksmith.