What does 'git reset' command will do without any other option? Will it reset the staging index with the head?
Without option, "git reset" is interpreted as "git reset --mixed HEAD".
git reset has three modes: soft, mixed, and hard (the default is "mixed").
Like many other git commands, git reset takes an argument which is a reference to a commit (a branch name, a tag name, a commit SHA, a relative reference like HEAD~2). By default, if no reference is specified, HEAD is used.
I suggest you read the last blog entry of "Progit", which explains in detail the "git reset" command: http://progit.org/2011/07/11/reset.html