To expend a bit:
The original ancestor germanic tongue had three consonants t d th (the latter often written þ or ð).
English and Icelandic preserved all three consonants as such (though in modern Icelandic, t is pronounced with strong aspiration, and d is pronounced more like t, but without aspiration).
German (and other closely related languages, like Yiddish, I think) turned t into z/ss (depending on position in the word), d into t, and th into d.
Other Germanic languages preserved t d, but turned th into something else (generally tor d).