There are two forms of selenium: Inorganic selenium, consisting of selenate and selenite, and organic selenium, consisting of selenomethionine and selenocysteine. According to the NIH, both forms can be good dietary sources of selenium.
However, the body more readily absorbs selenium supplements containing selenomethionine. According to the NIH, the body takes in more than 90% of selenium from selenomethionine, compared to 50% of selenium from selenite.