When video is interlaced, it means that each frame is broken up into two fields. This means that every other horizontal line of the video is recorded at a slightly different time than the other horizontal lines. The amount of time between the fields is half of the framerate, so in NTSC, the fields are recorded with approximately 1/60th second between them. Interlaced video was originally used to make old TVs appear less flickery, and although modern TVs do not have this problem, interlaced video is still used.
Most consumer camcorders only record in interlaced. Some higher-end camcorders have a 24p mode, which records progressive (uninterlaced) video at 24 frames per second, making it look more like film. Interlaced video can be undesirable when editing, or viewing on a non-interlaced screen, so it is often useful to deinterlace video.
It is sometimes confused that fields are the same as frames, and that 30fps interlaced video is the same as 60fps progressive video. They are not quite the same, because in interlaced video, every other line is actually recorded and played back from a lower starting position than the same line on the last field. When interlaced video is played back on a progressive screen, it appears flickery unless every other field is discarded.