Personally I'm a fan of using tabs for indentation in my code, but not all people feel the same way. I was recently contributing to a project that used four spaces as indentation. This causes a problem because my .vimrc says to use tabs. This can create some formatting problems obviously. Vim allows you to set simple commands inside the text the file you are editing using modelines. I was able to add:
// vim: tabstop=4 shiftwidth=4 expandtab autoindent softtabstop=4
as the last line of my file and now Vim converts my indentations to spaces (expandtab) for this one file only.