Eth is a letter used in some alphabets -- such as Old English, Middle English, Faroese, and Icelandic -- and is a symbol in the International Phonetic Alphabet. It represents the voiced interdental fricative, one of the two sounds that we associate with 'th' in English.
In English the sound represented by eth is found in words such as 'these', 'either', and 'bathe'. We also use 'th' for the sound known as a thorn (þ, Þ; or in the IPA, θ), which is the unvoiced interdental fricative, and is used in the words 'think' and 'bath'. The difference is hard for English speakers to hear, as we do not usually differentiate between these sounds. It may help to listen to the classic minimal pair for these sounds, thy (/ðai/) and thigh (/Þai/).
To use these symbols in HTML, you can use the codes Ð for capital eth (Ð) and ð for lowercase (ð). In unicode use U+00D0 and U+00F0. Eth may also be spelled edh or eð.
Eth should not be confused with the 'D with stroke', which appears in various languages. For example, in Vietnamese the letter Ð (lowercase đ) makes a /d/ with a glottal stop (/ʔd/). This same letter is used in some Slavic languages to make the /dʑ/ sound. It is not uncommon to find đ representing ð in older phonetic transcriptions, as đ is much easier to make on a standard typewriter.