Hotmail was a free web-based email service operated by Microsoft as part of Windows Live (now branded "Outlook.Com"). The email app with the very same names refers to this service as well.
What is Hotmail?
According to Wikipedia, hotmail was a free web-based email service operated by Microsoft as part of Windows Live, founded in 1996, which now is replaced by Outlook.Com (since February 18, 2013). There are also a few specific Android apps for this service, the "official" ones by "Microsoft + Seven" are Hotmail resp. Outlook.com.
What is this tag for?
If your issue is related to the Hotmail apps or the Hotmail service, this is the right tag to be used.
Do not confuse it with...
We also have the outlook tag. This is not referring to the mail service, but to the Windows application using the same name. So if your question is rather concerning this Windows desktop app (e.g. how to sync your Android device with the Outlook desktop app), use the outlook tag instead.