"Combing" or interlacing lines seen in a video clip.
Check the "Deinterlace Video" box. That is all.
MPEG Streamclip has a "deinterlace" box for you to tick off. If you notice that your video has those "comby" lines, it needs to be deinterlaced. Just import your (preferably uncompressed) video into MPEG Streamclip and when you are converting it to a web ready format, be sure to have the "deinterlace" box checked. Usually, most or all of the interlacing problem will be fixed.
The simplest way is to open it in MPEG Streamclip, Quicktime, or any other video player which allows you to advance through the video frame-by-frame (using the forward and back arrow keys on your keyboard). Find a spot which you suspect has more movement or action, and start moving through the video, frame by frame. Don't see any interlacing after looking for a while? It's probably not interlaced. You don't need to check the deinterlacing box.
FRAME RATES: You can also check frame rate of your video. In Quicktime, open your video and go to "Window >> Show Movie Info." It'll give you the frame rate (fps). In MPEG Streamclip, go to "File >> Show Stream Info."
If your video has the frame rate of 23.98 or 24, it's almost certainly not interlaced, but progressive. PAL DVDs (25 frames per second or fps) often are not interlaced, but you still need to confirm this visually. Sometimes when you get footage off a DVD made in a home DVD recorder, the interlacing will be there. It shouldn't be hard to miss. NTSC (29.97 fps) often is interlaced, but not always. The only foolproof way to know for sure is to simply look for the interlacing lines using the methods described here.
You'll see in the above screenshot of MPEG Streamclip (circled in blue) that it says "Deselect for progressive movies." There are other references to "interlaced" vs. "progressive" in MPEG Streamclip and also DivX.
Interlaced means, well, interlaced. It's got those comb lines. Progressive means that there are no lines. When you are preparing to encode your video, be on the lookout for any settings which give you a choice between "progressive" or "interlaced." If you've already deinterlaced your video and there are no more lines in it, consider it to now be "progressive."
This wonderful vidding tutorial (for Macs, but also informative for Windows users) gives a better overview of Progressive vs. Interlaced. I fully admit that I am not really clear on the nuances here!
Some more pro level video applications (like Final Cut Pro or Final Cut Express) have a deinterlace filter, which you can apply to individual clips. If you use this filter when editing your video, then you probably don't need to deinterlace again when you export the video out of your editing program or when you convert the video for the web.
Mac users will usually convert their video clips into a DV or MOV format before importing them into iMovie or Final Cut. If you see that your clips are from an interlaced source (you see the "combing" in the original source video), then tick the "deinterlace" box in MPEG Streamclip when you convert the clip to DV. You probably won't need to use any deinterlacing filters on it when you edit it, or after you export it. You should consider the clip to now be "progressive" unless you keep on seeing obvious interlacing lines.