How come one user can delete another user's 755 files?

The files are read-only; however, deleting a file doesn't modify it but only the parent directory (it basically removes the file from directory listing) – and it sounds like you have full write permissions to the directory.

You can set the sticky bit—aka “restricted deletion” flag—which will prevent anyone except the owner from renaming or deleting files in that directory (like in /tmp). To do this, run chmod o+t *directory* as the owner of the directory.

In a typical Unix filesystem, any file can be identified by an arbitrary number of directory entries, each of which holds a "hard link".

From an implementation standpoint there is a difference between deleting the last directory entry (hard link) for a file and simply deleting one reference out of of many. However, from a semantic standpoint there is no difference.

If multiple hard links exist to a file, writing to the file using any of them alters the file seen by all of them. Using rm on a link, however, merely causes the file to not be accessible via that link. Other links to the file continue to see the exact same file.