From Meta, a Wikimedia project coordination wiki
This template outputs a link to compare two pages or two revisions. For example:
"{{Compare pages|Template:Xpd1|Template:Xpd0}}"
gives "[1]" [2]
"{{Compare pages|21901107|22347634}}"
gives "[3]" [4]
"{{Compare pages|Template:Xpd1|22347634|Foobar}}"
gives "Foobar" [5]