Historically, TV CRTs are interlaced, while computer CRTs are not. Interlaced means that the electron beam skips every other horizontal line, filling in the missing lines on the next pass. A frame is composed of 2 fields. One field is all of the odd numbered lines, and the other field is all of the even numbered lines.
In a television display, interlaced scan refers to the process of re-assembling a picture from a series of video signals. The "standard" NTSC system uses 525 scanning lines to create a picture (frame). The frame/picture is made up of two fields: The first field has 262.5 odd lines (1,3,5...) and the second field has 262.5 even lines (2,4,6...). The odd lines are scanned (drawn on the screen) in 1/60th of a second, and the even lines follow in the next 1/60th of a second. This presents an entire frame/picture of 525 lines in 1/30th of a second. Analog NTSC video uses interlaced scanning, as do several digital television formats. Formats that include an "i" (1080i, 480i) use interlaced scanning. See also progressive scan.