http://en.wikipedia.org/wiki/File:Memory-card-comparison.jpg -- Public Domain
Digital memory is measured in bits and bytes. I am not going to get into a discussion of binary numbers and showing how to calculate in decimal and binary, but to understand computer memory and digital memory of all kinds, it is helpful to recognize the terminology and the relative sizes of the steps between different levels of storage media. Before showing the illustration, there are one or two points that need mentioning. First, 8 bits = 1 byte. It doesn't do any good to ask this question any more than it does to ask why there are 4 quarts in a gallon. There is a good historical reason which has long since been almost forgotten or never learned. The second is that commonly the real number of bytes at any level of storage device is rounded to a simple number. The progression of the numbers is usually indicated by a prefix on the word "byte" such as Kilobyte.
There is a simple progression here illustrated:
1 byte = 8 bits
1 Kilobyte KB = 1,024 bytes
1 Megabyte MB = 1,048,576 bytes
1 Gigabyte GB = 1,073,741,824 bytes
1 Terabyte TB = 1,099,511,627,776 bytes
1 Petabyte PT = 1,125,899,906,842,624 bytes
We'll knock it off there, suffice it to say the next three steps are Exabytes, Zettabytes and Yottabytes. In real life these steps are usually expressed much more simply by noting that each level is 1000 of the previous level, i.e. a kilobyte is a 1000 bytes and a megabyte is a 1000 kilobytes, and so forth. Although not strictly true, it makes the whole idea a little less complicated and is near enough for everyone except those who build computers.
OK, let's get on with the discussion of memory cards. They come in sizes expressed in the terminology above (there are no terabyte or petabyte memory cards, yet) Common memory cards today (2011) are usually around 2 to 4 Gigabytes (2GB to 4GB) Since this size card usually sells for less than $25 (some for much less) the discussion should probably end there. But it is interesting to note that the largest cards available today, either a 64GB SDXC card or a 64GB CompactFlash card, cost more than $250.