As the binary joke goes, "There are 10 types of people in this world; those who understand binary and those who don't."
So, why is this joke funny?
When reading binary numbers you start with the zero or one on the extreme right.
Let's use this example: 1001
Starting from the right last zero or one ("bit") the value is 1. If the right last bit was zero than the value would also be zero. Going one bit to the left (second to last zero or one from the right) the value of the bit would be 2 if there was a one but there is a zero so the bit place's value is zero. For each bit going right to left the place value doubles (1,2,4,8,16,32,64,128...).
Therefore, turning 1001 into a decimal number going right to left is:
8+ 0 + 0 + 1 = 9.
For the binary joke above:
10 = 2 + 0.
So "10 types" is read from binary to decimal as "2 types".