Trace elements are minerals that the body needs only in tiny amounts. Most of them are very important despite their low levels, and some don't seem to have much use at all (such as nickel, tin and vanadium). Generally speaking, if the body requires less than 100 mg of a particular mineral per day, it is considered a trace element.
Research is still exploring the role of various trace elements, and most don't even have a recommended daily allowance determined yet. A safe and adequate intake (SAI) amount or range may be listed instead, but most of these elements can be garnered from an average daily diet or standard multivitamin. Note that the levels of some of these trace elements found in food vary depending on the soil from which the food is grown.