Both CF and SD cards are memory cards.
Both CF cards and SD cards fall under the category of memory cards, specifically solid state memory cards. Solid state memory cards have no moving parts and thus are considered more reliable than hard disk drives.
Physical Size
A CF card is much larger than an SD card. While the CF card is roughly 1.5 square inches, SD cards are less than an inch long and about a half inch wide. The SD card is nearly flat, while the CF card is approximately a quarter inch thick.
Function
Both CF and SD cards are used to hold information, especially in digital cameras. Devices generally use one type of card or the other, but not both. CF cards tend to be used more than SD cards in digital cameras, especially in SLR cameras.
Memory Capacity
CF cards can generally hold more data than SD cards. Standard SD cards can only hold up to 2 GB of data, although SDHC cards are also available that can hold up to 32 GB.
Speed
The fastest SD card has a rough data transfer speed of 20 Mbps. The fastest CF card has roughly a 45 Mbps data transfer rate. The perceived difference in speed by a user, however, is generally negligible.
Interface
CF cards use a 50-pin interface, which allows for a 16-bit transfer bus (the amount of data that can be transferred at a time). SD cards use a 4-bit transfer bus and have only a 9-pin interface.
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