So... last question (I think): What makes an IFV, an IFV?
An APC is an armoured personnel carrier - a battle taxi, which takes infantry to where they're needed, at which point they dismount and fight normally. An IFV is an infantry fighting vehicle - the theory goes that it is more than a simple transport, but actually part of the infantry fighting team. Many designs (eg: BMP, early Bradley) had portholes so the infantry inside could use their weapons and fight from inside the vehicle. This was never very successful in practice, but the portholes are still retained.
In practice, however, the distinction blurs depending on how much money you have: an IFV for a rich country is a light tank or tank destroyer for a poor one, while an APC for a rich country is an IFV for a poor one. The M113 is a good example of this. In US service an APC; there were countless improvement programs to allow other countries to use it as something like an IFV. These ranged from moving the .50 cal HMG into an enclosed turret instead of a cupola, to mounting a 20mm autocannon on it, to putting the turret from a Scorpion light tank on the roof and calling it a fire support vehicle.
In general IFVs are more heavily armed and armoured, but as you can see the definition has a lot of flex in it.