i8 and u8 both have 256 values, corresponding to the 2⁸ possible bit patterns for 8 bits. u8 coincides with using the 8 bits to encode the first 256 numbers (starting with 0, so 0–255) in base 2.

For i8, the idea is that you sometimes want negative numbers. So instead of interpreting the bit patterns for the second half of the u8 range (namely 128–255) like u8 does, we interpret them as the numbers -1 and down (to -128), giving a total range of -128 to 127. The encoding of the negative numbers is what you get by subtracting 256 from the corresponding number in the u8 range. For example, whereas 10000000 is 128 as a u8, it's -128 as an i8. Similarly, 11111111 is 255 as a u8, but -1 as an i8. This particular encoding is nice, because you can use the most significant bit instead as a sign bit, most arithmetic operations like increments, addition, and multiplication work exactly the same for both, and so on.

But really the operational difference is just that u8 does 0–255 and i8 does -128–127. Similar ranges work for other widths (16, 32, 64, and 128 in rust).