The reason is that when referring to a flash memory card, the abbreviation "SD" means the "Secure Digital" card. It’s one of the types of flash memory cards. Others are XD, CF (or "Compact Flash"), and MemoryStick). These are different shapes with different connectors, and you just use whichever card fits the device you have (such as a camera).

That may be confusing, but it’s true!

When referring to video, "SD" means Standard Definition (also referred to as "SDTV" which is less ambiguous but implies that it is broadcast).

P.S.: Another answerer mentions this, and it’s important — be sure you have a fast enough card to handle the data rate. You won’t be able to re-use an SD card from an old digital still camera, because won’t be fast enough.

Newer SDHC (Secure Digital High Capacity, 2 GB and above) cards have "classes" of 2,4,6 and currently a top of 8, which refers to how fast they are able to write data. Newer devices can accept and use the older lower-capacity SD cards (not vice versa), but the bigger issue with HD video is the writing speed (besides the fact that you’d run out of space quickly with a 512MB card even if it were fast enough).

The manual for HMX-H100 does indeed specify class 6 (I checked it online myself). Someday all SDHC cards will probably be at least class 6, but be sure to check!

If you really want to cheap out on the cards at the expense of quality, the capacity listed for the different modes implies (by doing the math) that if a class 6 card is required for the "superfine" mode of 1080i (the highest quality the camcorder can do), then class 4 would be able to handle all modes of 720p, and class 2 would be sufficient for all modes of 480p.

When you are on vacation and the built-in memory fills, will you have a computer or some other external storage device to dump the video that’s filled the internal memory or is the plan to just stop recording – or delete video you have not transferred anywhere else? With removable memory, you still need to bring SOMETHING… whether is it more blank tapes or more SD memory cards, or something to dump the video to…

Read the camcorder manual… All the AVCHD camcorders using SD memory require Class 6 SD memory cards. These are not as cheap as the class 2 cards used in still cameras. They need to be fast because the need to keep up with the video data stream.
http://www.samsung.com/us/consumer/detail/support.do?group=camerascamcorders&type=camcorders&subtype=highdefinition&model_nm=HMX-H100&language=&cate_type=all&mType=UM&dType=D&vType=&cttID=2237770&prd_ia_cd=04020600&disp_nm=HMX-H100&model_cd=HMX-H100N/XAA&menu=download&menu2=detail
Refer to page 30.
An SD flash memory card (in this case SDHC) is "Secure Digital High Capacity" and has nothing to do with High Definition video or Standard Definition video. Failure to use a fast class 6 SD card will likely result in dropped frames and reduced video quality.

And "easy to edit" is a relative term. High definition video is high definition video, no matter how you slice it. It uses up to 4 times more space on your computer hard drive. It is VERY RAM and CPU intensive. Go to page 131 of the manual… this records high definition to h.264 (MPEG4-AVC)… This is not easy to edit and your MovieMaker won’t be able to deal with it (if you want to stay in high definition, then Sony Vegas and Adobe Premiere are the Windows/Vista suspects; iMovie ‘08 or newer running on an Intel Mac or FinalCut running on an Intel Mac are required.