Even more confusing than the process of deciding which model of digital camera to buy is the process of selecting a memory card for your camera. This is compounded by the fact that often your first digital camera will not come bundled with a memory card, and you will need to purchase it separately.
Important characteristics for a memory card, beyond or not it is compatible with a given digital camera model, are the file storage capacity (generally measured in gigabytes), and the file transfer rate (the higher the better). As an example of the numbers involved, a 6 megapixel resolution camera can store about 320 high resolution JPEG images on a 1GB memory card. A 2GB memory card would hold 640 images, and so on. But note that if you store images in the camera's RAW image format as well, your memory card will accept a significantly smaller number. This is why high-capacity memory cards are favored when you can afford them.
The wide variety of memory card types and makers reflect the relative immaturity of the field of digital photography. The different companies are still battling for market dominance, and no standard has yet been established for memory cards.
However, there are currently two main types of memory card that seem to stand above the others. These two types are known as Compact Flash (or CF) and Secure Digital (SD). For the rest of this article I shall restrict my discussion to these two card types. Most digital cameras will support only one memory card type, although the very high end digital SLR models, like the Canon EOS-1Ds Mark III and the Nikon D3 support both CF and SD memory cards for maximum flexibility.
Compact Flash, which is currently the most popular memory card type for digital cameras (due to its historically superior capacity and reliability), was introduced by SanDisk in the mid nineties, so it has been around for a while. All the major digital camera brands employ CF memory cards at at least some, if not most, of their models. SanDisk produces memory cards that can hold up to 8 GB in file size.
For people who need to reel off a great number of shots in order to get the one or two great ones (sports photographers come to mind here), the high capacity cards are extremely convenient. Transfer rates are specified in term of the incremental "1x" rate of 150 KB / s. A "12x" card would there before be capable of a maximum file transfer rate of 1800 KB / s. CF memory cards come in one physical size only, though with two possible thicknesses, designated as Type I and Type II, with the second being the thicker. A Type II memory card will not fit into a Type I slot, so be sure to get the correct Type when selecting CF memory cards.
Because CF memory cards are reliably large (1.43 inches by 1.68 inches) they are less likely to be found in use with smaller point-and-shoot digital camera models. Instead, their physical size makes them more capable to the digital SLR models.
The Secure Digital memory card name derives from the fact that it was originally introduced as a means to secure store music files. The idea was that music files would be copyright protected and limited access would be allowed to files stored on SD disks. But the concept was short-lived when the security protocol was cracked not long after its introduction. Because of the tight association with the music industry, slots that accept SD cards also accept other devices like Bluetooth antenna, PDAs, mobile phones, and so on.
SD memory cards are used by all the top digital camera brands, including Casio, Canon, Nikon, Pentax, Kodak, Panasonic, and Konica Minolta.
Like CF memory cards, the SD card transfer rate is measured in the same units of 150 KB / s. Due to the smaller physical size of the SD memory card (1.26 inches by 0.94 inches) the maximum file storage capacity has historically been a great deal lower than for CF cards. Typically around 1 GB. However, SD has now targeted the high capacity market, and to do so it has had to shift to a new file format. Unfortunately this was done without a corresponding change in physical dimensions of the card, which has meant that older cards are often inserted into newer SD card slots that can not read them, and vice versa, causing some confusion to consumers.
Once you have a memory card that is full of images, you are going to want to transfer the files to your computer, where you might process them with a software application like Photoshop, or you might send them on to someone else for the editing. One common way to transfer images from your memory card is to use a card reader. This is a small device, roughly of ipod size, which contains one or more memory card slots at one end, and a cable at the other end which connects to the USB port of your computer. In this way, your card reader simply becomes another port from which to read data onto your hard drive.
If your digital camera does not ship with a card reader, it almost certainly is accommodated by a cable which can be used to connect the USB port of your PC to the camera. In this way, your camera doubles as the card reader device that you might otherwise have used to read the memory card.
Other devices, such as photo printers, are likely to have memory card slots built right into them, so that they can accept a memory card directly. Some devices are even capable of receiving a WiFi signal sent from a camera so that files can be transferred "over the air".
No matter what the model of your digital camera may be, it is always a great idea to first consult the user manual to see what your options are regarding memory cards. Never make the assumption that one memory card is likely to work for your camera simply because a sister model, or immediate predecessor model, uses the same memory card. It is relatively the case, but it pays to find out before you order new cards.
You can also visit the SanDisk site and check memory card / camera model compatibility. SanDisk lists all the major camera manufacturers and for most current camera models, gives the SanDisk cards that can be used with them.