The European standard for television (PAL) is 25 frames per second (for North America, Japan and some other places it is 30 fps.) In other words every 25th of a second a different frame is displayed on the TV screen. Each of these frames is made of two fields, an upper and a lower. Each field is every second line of the image so the upper field is lines 1,3,5,7,9,11,13,15,etc and the lower field is lines 2,4,6,8,10.... up till 576. These two fields are a 50th of a second apart: we have 50 half images displayed/recorded per second.

When you capture a still frame on the computer this is what you get: two fields making one frame. If any degree of motion has happened between the 2 fields the image will look stripy. Some video cameras can be set to a mode called progressive scan where the 2 fields are taken at the same time and thus the image has no difference between the 2 fields. Some people describe the resulting video as looking more 'filmic'.