Three plug types cover a huge share of the world's sockets, and travellers mix them up constantly. Here's the plain-English difference, and how they relate to each other.
Two flat parallel pins, no earth. Used mainly in North America (USA, Canada, Mexico) and Japan, on 100-127V supplies. A Type A plug physically fits a Type B socket, since Type B is just Type A with an added earth pin.
The same two flat pins as Type A plus a round earth pin below. Standard in the USA and Canada for anything that needs grounding, at 120V. A Type B socket accepts both Type A and Type B plugs; a Type B plug will not fit a plain Type A socket because of the earth pin.
Two round pins, no earth, known as the "Europlug". This is the single most widespread plug in the world: it fits sockets across nearly all of continental Europe, plus much of Asia, Africa and South America, on 220-240V. A Type C plug slots into Type C, E, F and several other round-pin sockets, which is why one Type C adapter covers so much of the globe.
The key point for travellers: A and B belong to the low-voltage flat-pin world (North America, Japan), while C belongs to the high-voltage round-pin world (Europe and beyond). A flat A/B plug will never fit a round C socket, and vice versa, so crossing between these two worlds always needs an adapter. And because the voltages differ (roughly 120V versus 230V), single-voltage appliances also need a converter when crossing.
No. Type A is two flat pins (North America, Japan); Type C is two round pins (Europe). They are not compatible, so you need an adapter to cross between them.
Type C, the round-pin "Europlug", is the most widespread, followed by Type A. Between them they cover a large share of the world's sockets.
Almost. Type B is Type A with an added round earth pin. A Type A plug fits a Type B socket, but a Type B plug will not fit a plain Type A socket.