Compression Help

Sometimes I feel I understand compression. I understand what it is used for, and when, but, when it is time for compression, i usually rely on presets. I understand input, threshold, and out put gain, but ratio, knee,attack, and knee all baffel me. I have read a lot about them but i think i need some sort of visual explanation, or at least a written exsample of where you would use these functions. Is there maybe a free video tutoriol that could help me understand more. Maybe one on eq too.
Sometimes I feel that everything I know is incorrect.