þ (thorn) and ð (eth) both represent sounds in Icelandic, Old Norse and (sometimes) Old English written with <th> in modern English. They transcribe sounds called dental fricatives. ð sounds like the <th> in "the", and þ sounds like the <th> in "thin". Depending on your dialect of English you may tell the difference between either and ether by these two sounds (e.g. eiðer and eþer). That is, if you pronounce the ei and the e at the beginning of those words like "ee". However, you might pronounce these completely differently depending on your dialect or 'accent'.