Subject: What is site-directed mutagenesis?

I was reading an article on cellulase enzyme and came across the terminology 
site-directed  mutagenesis as a method to improve the low specific enzyme 
activity of some microorganism.  I would like to know what that means as well 
as how does it work.  Also what is meant by specific enzyme activity? and is 
site-directed  mutagenesis the right way to go? and do you know of any other 
way to increase the specific enzyme of microorganisms?  Thanks. 

Re: What is site-directed mutagenesis?