Bangs, derived from Bang path, describe [alternate] versions or viewpoints of things. They are easily distinguishable by the exclamation point [!] separator, such as in Hufflepuff!Harry.
The terms preceding the exclamation point show what is different or has changed, and the terms after show to where the changes are applied. If the latter simply states "AU", the changes apply to the entire scope of the product(?).
There is no space between the two sides and the exclamation point, but either side can have spaces between multiple terms (as opposed to CamelCase). Whether or not each side is capitalized as a new sentence is up to the user.
Alts are usually not applied to explicitly canon cases. For example, it would be pointless to give a fic the "Hogwarts student!Harry" alt, unless the author need to point out that the scope of the fic is of Harry's student days, as opposed to his adult or younger child years.
In contrast, for most ships the Dating!<ship name> alt applies. On the wiki, to denote that a particular non-explicitly-canon parameter is notable, one can add the appropriate alt next to it.