IBM网站上给出的XML的介绍（1）不完全摘录
Tags, elements, and attributes
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There are three common terms used to describe parts of an XML document: tags, elements, and attributes. Here is a sample document that illustrates the terms:
<address>
<name>
<title>Mrs.</title>
<first-name>
Mary
</first-name>
<last-name>
McGoon
</last-name>
</name>
<street>
1401 Main Street
</street>
<city state="NC">Anytown</city>
<postal-code>
34829
</postal-code>
</address>
[*]A tag is the text between the left angle bracket (<) and the right angle bracket (>). There are starting tags (such as <name>) and ending tags (such as </name>)
[*]An element is the starting tag, the ending tag, and everything in between. In the sample above, the <name> element contains three child elements: <title>, <first-name>, and <last-name>.
[*]An attribute is a name-value pair inside the starting tag of an element. In this example, state is an attribute of the <city> element; in earlier examples, <state> was an element (see A sample XML document).
上面的内容表明了XML比HTML要严格了许多，而tag和element之间的关系是“数据元素”和“数据项”之间的关系，之所以把上面英文解释中的几个东西叫做element，是因为它们实现了tag==name的完整性定义。
以前的XML中在tag中叫做“element”的东西，随着技术的进步，现在被叫做“attribute”，因为它们并没有实现数据的完整性定义，而只是作为XML中tag的一个属性来实现的。
How XML is changing the Web
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Now that you've seen how developers can use XML to create documents with self-describing data, let's look at how people are using those documents to improve the Web. Here are a few key areas:
[*]XML simplifies data interchange. Because different organizations (or even different parts of the same organization) rarely standardize on a single set of tools, it can take a significant amount of work for applications to communicate. Using XML, each group creates a single utility that transforms their internal data formats into XML and vice versa. Best of all, there's a good chance that their software vendors already provide tools to transform their database records (or LDAP directories, or purchase orders, and so forth) to and from XML.
[*]XML enables smart code. Because XML documents can be structured to identify every important piece of information (as well as the relationships between the pieces), it's possible to write code that can process those XML documents without human intervention. The fact that software vendors have spent massive amounts of time and money building XML development tools means writing that code is a relatively simple process.
[*]XML enables smart searches. Although search engines have improved steadily over the years, it's still quite common to get erroneous results from a search. If you're searching HTML pages for someone named "Chip," you might also find pages on chocolate chips, computer chips, wood chips, and lots of other useless matches. Searching XML documents for <first-name> elements that contained the text Chip would give you a much better set of results.
I'll also discuss real-world uses of XML in Case studies.
上面提到了3点内容
（1）使用XML能够显著提高数据转换的工作
（2）能够是编码的工作在相当的程度上减少，使代码更加规范，提高计算机在“较少人为干预”的情况下生成规范的文档
（3）使用XML对于企业用户来说，能从某种程度上提高被搜索引擎找到的可能（说白了：越规范排名越可能靠前）
One important point about XML documents: The XML specification requires a parser to reject any XML document that doesn't follow the basic rules. Most HTML parsers will accept sloppy markup, making a guess as to what the writer of the document intended. To avoid the loosely structured mess found in the average HTML document, the creators of XML decided to enforce document structure from the beginning.
XML文档首先要做的是整体规划和部署，要根据XML规范的约定来进行一个“词法分析+语法分析”的工作，而分析的结果将会是“高确定性的”；而HTML在这方面就不可能，它首先会有比较臃肿和不严格的tag，这样，也导致了它的“高不确定性”。
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