in git when I specify a parameter, ie
git log -n 5
what is the difference of using a parameter with one dash \"-\" as opposed to
That's not really git specific. Many programs use the following convention:
This is handy, because it allows you to specify many single-letter parmeters at once with a single dash and all letters of the parameters you need: ls -al is equivalent to ls -a -l.
Often, one-letter parameters are the most used ones and can have a longer equivalent with two dashes: for example git add -v and git add --verbose mean the same.