A tag can be a single word or a phrase (which can include spaces). A phrase does not need to be surrounded by quotes: just type it as it is.
To search using more than one tag, each tag *must* be separated by a comma (we need the comma as spaces are allowed in tag phrases). For example:

To search for Hawkmoon OR Elric, type the following and then press "search"
Hawkmoon, Elric
To search for Hawkmoon AND Elric, type
Hawkmoon, +Elric
To search for Hawkmoon NOT Elric, type
Hawkmoon, -Elric
To search for Hawkmoon OR Elric but NOT Behold the Man, type
Hawkmoon, Elric, -Behold the Man

Note: if you type a + or - symbol before the first tag, it will be always be ignored