Well I can help you with key frames at least. Say your character has his arm straight out in frame 1. And then you go to frame 10 and make his arm stick up in the air. The program will automatically "fill in" all the frames in between - so from 1 to 10 he will raise his arm. Now a key frame is simply a frame that won't be automatically "filled in" like this. Say that you make frame 10 a key frame. And then you go to frame 20 and put his arm back to how it was in frame 1. The program will now "fill in" from 10 to 20 - result, he raises his arm and then lowers it again. If frame 10 were not a key frame, then there would be no movement at all, because the program would "fill in" from 1 to 20.
Note that a frame can be a key frame for one object or body part but not for another. In the above example, you might make frames 1, 10, and 20 key frames for his arm, but not for the rest of him.
Also note that if you move anything, the program automatically makes that frame a key frame for that body part. Thus, moving the figure's arm in frame 10 means that frame 10 is automatically a key frame for his arm. So you can then go straight to frame 20 and move it again, knowing that the position in frame 10 is "saved". You can make other frames key frames as well by selecting on them in the animation window (that thing with all the green squares) and clicking the plus sign. Key frames are shown in brighter green. As you can see, the frames are ordered from left to right, and the objects in the scene from top to bottom, creating a grid. Each square represents an object in a frame, and it can be key or not.
If an animation over many frames is a journey, then the key frames are the waystations along the way. If you see what I mean.