Binary Coded Decimal (BCD) is a method of storing numbers. It is suitable for making it easier for humans to read binary data, but not much else in a theoretical sense.

Each digit in the decimal number is encoded in 4 bits of standard binary.

0  0000
1  0001
2  0010
3  0011
4  0100
5  0101
6  0110
7  0111
8  1000
9  1001

Hence, the number 69 would look like
0110 1001
in BCD.

If you look at BCD data in hexadecimal notation, it looks just like regular decimal notation. Duh. BCD is just hex with A-F ignored. Just remember that they don't add or multiply the same...

If you run Gnome, you can see BCD in action, with Gnome's lovely useless Binary Clock applet.