Text files can be tagged for reference to sound or video files which you can hear or see. For example, a text might contain something like this: blah blah blah ...<a href=http://gandalf.hit.uib.no/c/l/32401-1.mp3> blah blah etc. A concordance on blah blah could pick up the tag so you can hear the source mp3 file. See defining multimedia tags.
These tags are often used to signal the part of speech of each word; they're also widely used in HTML, XML, SGML for "switches", e.g. <H1> to switch on Heading 1 style and </H1> to switch it off again. You should use the same opening and closing symbols, usually some kind of brackets, for all your tags (as the British National Corpus does using SGML or XML markup): <Noun>,<Verb>,<Pronoun>.
(A particular sub-variety of tags within 2 separators sometimes used is tags with underscores at the left and space at the right like this