What is &amp used for

& is HTML for "Start of a character reference".

& is the character reference for "An ampersand".

&current; is not a standard character reference and so is an error (browsers may try to perform error recovery but you should not depend on this).

If you used a character reference for a real character (e.g. ™) then it (™) would appear in the URL instead of the string you wanted.

(Note that depending on the version of HTML you use, you may have to end a character reference with a ;, which is why &trade= will be treated as ™. HTML 4 allows it to be ommited if the next character is a non-word character (such as =) but some browsers (Hello Internet Explorer) have issues with this).