Positional Numeral Systems
In a positional numeral system, a digit's value depends on both its face value and where it sits in the number. Any integer written in base b is a polynomial expansion, N = Σ_(i=0)^(n-1) d_i · b^i, where d_i is the digit at position i. That's why the digit "2" means something different in "25" than it does in "52". Its place, not just its face value, sets its contribution.
The same quantity can be written in any positive integer base. Converting decimal to another base means repeatedly dividing by the target base and reading the remainders in reverse order; converting the other direction means expanding the polynomial above and summing the terms.