<DT> is used in conjunction with <DL ...> and
<DD> to create definition lists. <DT> is usually used as the first part of a <DT>/<DD> combination. So, for example, this code creates three pairs of terms and definitions for the terms.
this code
produces this
<DL>
<DT>flame
<DD>an ugly argument in a newsgroup
<DT>spam
<DD>annoying unrequested email
<DT>troll
<DD>someone who start flames
by posting stupid things
</DL>
flame
an ugly argument in a newsgroup
spam
annoying unrequested email
troll
someone who start flames
by posting stupid things
<DT> is a container, so you can put a </DT> at the end of the term. However, the end tag is optional and is rarely used.