Often those are file/page fragments that are to be included in other files/pages, but it could mean any number of things, really. Where did you see them, and what did you notice about how they're used? [ October 09, 2008: Message edited by: Ulf Dittmer ]
The official extension for files to be included in other JSP files is .jspf (which means JSP fragment).
We previously named our included files ".inc", but it didn't work well, because the IDE didn't automatically recognise those as JSP files, so syntax highlighting would not work on those files. We renamed the files to ".jspf" and now it works as it should.