011101 = 0 x 2^5 + 1 x 2^4 + 1 x 2^3 + 1 x 2^2 + 0 x 2^1 + 1 x 2^0 = 16 + 8 + 4 + 1 = 29
The same positional notation is used with decimal numbers, except using powers of 10.
With two's complement notation, all integers are represented using a fixed number of bits with the leftmost bit given a negative weight. So 100011 would be
100011 = -1 x 2^5 + 0 x 2^4 + 0 x 2^3 + 0 x 2^2 + 1 x 2^1 + 1 x 2^ 0 = -32 + 2 + 1 = -29
There are a number of nice features to two's complement representation. The first is that the normal rules used in the addition of (unsigned) binary integers still work (throw away any bit carried out of the left-most position). Second, it's easy to negate any integers: simply complement each bit and add 1 to the result (011101 complemented is 100010 plus 1 is 100011; 100011 complemented is 011100 plus 1 is 011101). Finally, the left most bit tells you if the integer is positive (0) or negative (1).
The range of integers and the ordering does change. For example, four bits can represent the unsigned integers values 0 (0000) through 15 (1111). However, four bit two's complement representation represents the signed values - 8 through +7 ordered as below
1000 1001 1010 1011 1100 1101 1110 1111 0000 0001 0010 0011 0100 0101 0110 0111 -8 -7 -6 -5 -4 -3 -2 -1 0 1 2 3 4 5 6 7