This site defines a logarithm and has practice worksheets.

http://www.math.utah.edu/~alfeld/math/log.html

These sites give an overview of logarithm:

http://hyperphysics.phy-astr.gsu.edu/hbase/logm.html

Review logarithms at this interactive site:

http://academic.pgcc.edu/~ssinex/rad_decay.pdf