A logarithm answers the question: "To what power must I raise the base to get this number?"
If by = x, then logb(x) = y.
For example: 23 = 8, so log2(8) = 3.
To convert between bases, divide the log in the target base by the log of the original base in the target base:
logb(x) = ln(x) / ln(b) = log10(x) / log10(b)
The calculator handles this conversion automatically — just enter your number and desired base.