From "A Gentle Introduction to XML":
The technical term used in XML for a textual unit, viewed as a structural component, is element. Different types of elements are given different names, but XML provides no way of expressing the meaning of a particular type of element, other than its relationship to other element types. That is, all one can say about an element called (for instance) <blort> is that instances of it may (or may not) occur within elements of type <farble>, and that it may (or may not) be decomposed into elements of type <blortette>. It should be stressed that XML is entirely unconcerned with the semantics of textual elements: these are application dependent. It is up to the creators of XML vocabularies (such as these Guidelines) to choose intelligible names for the elements they identify and to define their proper use in text markup. That is the chief purpose of documents such as the TEI Guidelines. From the need to choose element names indicative of function comes the technical term for the name of an element type, which is generic identifier, or GI.
Within a marked up text (a document instance), each element must be explicitly marked or tagged in some way. This is done by inserting a tag at the beginning of the element (a start-tag) and another at its end (an end-tag).12 The start- and end-tag pair are used to bracket off the element occurrences within the running text, in rather the same way as different types of parentheses or quotation marks are used in conventional punctuation. For example, a quotation element in a text might be tagged as follows:
... Rosalind's remarks <quote>This is the silliest stuff that ere I heard of!</quote> clearly indicate ...
As this example shows, a start-tag takes the form <quote>, where the opening angle bracket indicates the start of the start-tag, ‘quote’ is the generic identifier of the element which is being delimited, and the closing angle bracket indicates the end of a tag. An end-tag takes an identical form, except that the opening angle bracket is followed by a solidus (slash) character, so that the corresponding end-tag is </quote>.13