Ä or ä is one of the 4 extra letters used in German. It can be replaced by using the letters Ae or ae. In English language newspapers it is often written as A or a but this is not correct. Ä and ä are usually pronounced like the "e" in "bed".
Internet addresses are written as "ae" because the internet address system can only understand ordinary English letters.
German is one of the official languages of Switzerland, but people from Switzerland who speak German do not use the extra letter, they always use ae.