Notions: video is basically a three-dimensional array of color pixels. Two dimensions serve as spacial (horizontal and vertical) directions of the (moving) pictures and one dimension represents the time domain. A frame is a set of all pixels that (approximately) correspond to a single point in time. Basically, a frame is the same as a (still) picture. However, in interlaced video, the set of horizontal lines with even numbers and the set with odd numbers are grouped together to fields. The term picture can refer to a frame or a field.
Method: Video data contains spatial and temporal redundancy. Video compression typically reduces this redundancy using lossy compression. Usually this is achieved by: