This is a backup copy of your account, updated nightly. You can do this with the following command, which will list all the files and directories in the directory that you're in, sorted by their total size. Compress large files that you do need. The gzip command, in the sipb and gnu lockers, has a good compression rate. You should delete them yourself.
Core files are useful for finding out what caused the program to fail, but little else.
They can be safely deleted. These are object files that you get when compiling C or Fortran programs. You can regenerate them by recompiling your programs, and so they can be safely deleted. These files are pictures and movies taken by the SGI camera, respectively. This directory is created by the Netscape web browser, and can occupy up to 5Mb of your disk quota.