standard/ holds the standard stylesheets. These are used no matter what skinthe user selects. If the user selects the "Classic" skin, then *only* thestandard/ stylesheets are used.
contrib/ holds "skins" that the user can select in their preferences. skinsare in directories, and they contain files with the same names as the files inskins/standard/. Simply putting a new directory into the contrib/ directoryadds a new skin as an option in users' preferences.
custom/ allows you to locally override the standard/ and contrib/ CSS. If youput files into the custom/ directory with the same names as the CSS files inskins/standard/, you can override the standard/ and contrib/ CSS. For example,if you want to override some CSS in skins/standard/global.css, then you shouldcreate a file called "global.css" in custom/ and put some CSS in it. The CSSyou put into files in custom/ will be used *in addition* to the CSS inskins/standard/ or the CSS in skins/contrib/. It will apply to every skin.