DTD (Document Type Definition): DTDs comprise the rules data must adhere to for it to be valid. You declare element names and their valid hierarchical structure, as well as their attributes and entities, in the DOCTYPE processing instructions. Entities include ELEMENT, ATLIST, and ENTITY. DTDs enable you to define the complete set of XML markup elements that developers can use in a project.

HTML (Hypertext Markup Language): HTML is the methodology for publishing hypertext on the World Wide Web. It is a nonproprietary format based on SGML; you can create and process HTML using a wide range of tools, from simple plain-text editors where you type in HTML code manually to sophisticated WYSIWYG authoring tools that automate much of the toil in creating Web pages. HTML uses tags such as <h1> and </h1> to structure text into headings, paragraphs, lists, hypertext links, and so on.

One key attribute that distinguishes HTML from XML is that HTML is not self-aware.

Metadata: Metadata is data that describes other data. For example, recordset metadata describes the recordset, and includes information such as the number of columns, the data types of those columns, their names, and so on. An application requires metadata for most recordset operations. For example, it might use the data type of a column to determine what kind of variable to bind to that column, and the byte length of a character column to determine how much space it needs to display data for a given column. The application itself determines the metadata for a column. In this project, metadata about the XML document is described in the DTD or in the XML document itself; this metadata enables other applications to query and manipulate it.

SGML (Standard Generalized Markup Language): SGML is an international standard that describes the relationship between a document's context and its structure in an open and vendor-neutral format. SGML lets you share information across computer platforms and applications. Typical uses for SGML range from electronic publishing to document repositories.

XML is a subset of SGML designed specifically for the Web.

SQL-DMO (Structured Query Language-Distributed Management Objects): SQL-DMO is part of a comprehensive framework called SQL Distributed Management Framework, an integrated framework of objects, services, and components for managing Microsoft SQL Server. VB lets you add a reference to the SQL-DMO object library and gain programmatic access to all the objects SQL Server lets you manipulate. You can learn more about its programmable objects by using VB's object browser to view the SQL-DMO object model.

XML (Extensible Markup Language): XML is a markup language for documents containing structured information. Structured information comes in several types. For example, SQL Server contains structured information, as do a structured storage file and a recordset.

What makes XML special? Nothing, actually. Web developers want a universal methodology to represent, manipulate, and exchange data, and XML provides this. Other technologies, such as ActiveX Data Objects (ADO), provide this too. However, ADO and similar, competing technologies are proprietary. XML's open standard means that if application A uses ADO to communicate data, and application B uses a proprietary home-grown system to communicate data, they can both agree to use XML to describe and communicate data to each other, regardless of the technology they use internally. Developers previously used a comma-delimited format to exchange data; however, XML marks a big step forward because the data contained in a comma-delimited format is not self-describing and cannot be queried or manipulated.

XML lets you communicate and exchange data with other applications. It would be difficult to overstate the importance of this feature. A unified, structured way to exchange data makes everyone's life easier. At this time, a lot of ISVs are waiting to see who will jump on the XML bandwagon before they commit themselves. But we're telling you now: XML has more than hype, momentum, and potential going for it; it solves many of the more difficult problems you face every day.

XML Query Language: XML Query Language enables you to query the structured data XML holds. The XML viewer does this for you in the VBPJ Special Project, but what developers really need is a standard mechanism to query XML documents similar to the way you query database tables. Several proposals are vying to serve as the standard, but you must resort to XML viewers such as the one in this article until the development community standardizes on such a language to query XML documents.

XSL (Extensible Style Language): XSL is a style-sheet language that defines the rules for displaying structured XML data. XSL enables developers to generate a presentation structure that might be quite different from the original data structure. For example, it lets you format and display an element in multiple places on a page or not display it all, depending on your needs-all without complex scripting. XSL is compatible with and plays a complementary role to CSS. You use CSS to format HTML by applying formatting information to a "view" of data, and XSL to transform XML data into HTML.