On other sites, for example SuperUser, tags with these two names are synonymous.
Well, technically wlan is a broader term than Wi-Fi which just happens to be a brand name... and, uhm well, describing the (by far) most widely adopted standard.
Interestingly Electronics.SE does not even have a wlan. Unix&Linux does not synonymize wlan with wifi - though they have no tag definition for wlan either, use it only sparringly, and often double-tag with wifi.
In practical terms, since everybody is using Wi-Fi as a general term for wlan anyways, we probably don't lose much in synonymizing those two tags.