I am trying to make my own Arduino unit with instructions from the site.  However, I find that the memory of the Arduino microcontrolers is very small. I heard that you can expand the memroy with an SD card.  Is this true?  By memory I mean the memory for codes and instructions. Furthermore, what is the difference between the code that Arduino uses and C++?  I know C++ but not the one that Arduino works on.
And finally, do Arduino units need to be connected to computers to work because I know that they're USB slave, but can they function on their own.  That part is kind of vague.
Thanks a lot!!