Describe two's complement.

Two's complement is improvement on one's complement.
To get a negative number in One's complement, one has to invert all numbers (turn 1 to 0, and 0 to 1), for two's complement is the same but add 1 (e.g. in 4 bit, 0001).
For example to get a negative -5 in 4 bit:Positive 4: 0100₂Invert positive 4: 1011₂, this is negative -4 in one's complement.Add 0001 to inverted positive 4: 1100
A simple way to read the two's complements binary number: usually, from left most to right we see each bit as [8, 4, 2, 1], but for two's complement we (humans) can read it as [-8, 4, 2, 1], so 1100 would be -8 +4, which is -4.
Though, two's complement is more common for two reasons.In one's complement there are two representations for 0 (in 4 bit it would be 0000 and 1111).When adding a positive and a negative number you also need to add 1 (e.g. in 4 bit, 0001) after the operation.