Open . == Start .

If you're used to Windows command shell cmd.exe, you probably execute start . to start explorer with current directory; very nice saves a few seconds from the tedious navigation by hand.

In Mac OS X, the same thing can be done with open . or open /some/directory. Very handy.

Some variations of the open command

open . -g  : opens finder in the background

open -e filename : open filename with textedit { -t for default text editor and using -a you can specify application }

open filename : will open with default app

obviously the man pages have much more ...