The sound (θ) (TH unvoiced) is unique to the English language. To pronounce it, place the tip of your tongue on the tip of your upper teeth and blow air through the space between the tongue and the teeth, without vibrating your vocal cords. The sound (θ) is close to the (s) sound. It is found in words such as bath, path, youth, breath, and sixteenth.
The sound (ð) (TH voiced) is also unique to the English language. It is also pronounced by placing the tip of the tongue on the tip of the teeth and blowing air through the space between them, but this time using the vocal cords. It is close to the (z) sound and may be found in words such as the, then, that, these, they, and thus.
(θ) TH Unvoiced (ð) TH Voiced