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IFV usually have a crew of 3 (Driver, Gunner/Loader, Commander) while APC usually have a crew of 1-2 (Driver, Commander/Gunner/Loader). Though the resposibilities will differ depending on the model and country. The APC will often be able to transport an entire squad while the IFV will mostly only be able to transport a fire team because the addition of a turret means less space in the rear compartment.
Though these are not really clear cut definition but in general an [APC](http://upload.wikimedia.org/wikipedia/commons/d/d5/US_M113_in_Samarra_Iraq.jpg) is an armoured vehicle with a light armament (usually just an MG) whose main responsibility is getting infantry from point A to point B so that the infantry can dismount and fight on foot.
While an [IFV](http://upload.wikimedia.org/wikipedia/commons/b/b0/2008_Moscow_May_Parade_Rehearsal_-_BMP-3.JPG) is an armoured vehicle with a slightly larger armament (Usually an automatic cannon) whose purpose is not just to safetly transport infantry it is also well armoured and armed enough to be able to fight alongside the infantry or keeping the infantry mounted protecting them so they can assault alongside tanks and break through the enemy lines.
Though there are a lot of vehicles that won’t stay neatly in either of these categories this is however a helpful rule of thumb.