Data Exchange Formats, Specifications and Standards
HTTP - Hypertext Transfer Protocol. According to the World Wide Web consortium, "The Hypertext Transfer Protocol (HTTP) is an application-level protocol for distributed, collaborative, hypermedia information systems." It is the communication protocol for exchanging web pages written in HTML markup language. See http://en.wikipedia.org/wiki/Hypertext_Transfer_Protocol
Markup Language - Markup languages use tags to give meaning to parts of a document. HTML uses tags mostly to denote formatting, e.g. the HTML snippet <h1>This is a First Level Heading</h1> would indicate that the text between the opening (<h1>) and closing (</h1>) tag elements should be formatted in the style for heading 1. Other markup languages, such as SGML or LaTex may provide more semantic or descriptive tags, e.g. <abstract>This is the abstract to this article</abstract>. Such tags may cue formatting but also describe the role and give meaning to a component of a document. Still other markup languages such as XML use tags to encode data elements, e.g. <body_weight_in_kg>23</body_weight_in_kg> See http://en.wikipedia.org/wiki/HTML.
HTML - Hypertext Markup Language. A standard markup language for creating web pages maintained by the world wide web consortium. HTML5 is the latest HTML specification. See the entry for Markup languages below. Also see http://en.wikipedia.org/wiki/HTML
CSS - Cascading Style Sheets provide a mechanism to control the appearence and formatting of HTML, e.g. CSS sheets can describe the appearance of the bullets used in an unordered list or the color and formatting for hyper-links. Whereas an HTML tag such as <h1> denotes a first level heading it is the CSS that determines the font and text style of the heading.
XML - Extensible Markup Language. The XML markup langauge is designed to be user extendable, i.e. it allows users to define their own tags (also called elements). Element definitions and their relatioships are defined in an xml schema. XML is a common data exchange format because it is both human and machine readable and because it is very flexible. XML is often used to exchange data in human readable text files. See http://en.wikipedia.org/wiki/XML.
Semantic Web - The semantic web is the next iteration of the world wide web, one that features information that is both human readable and machine readable. HTLM pages are easily read by people but they are difficult for machines to read reliably because so much of normal language requires context and has vague meanings that computers are lousy at deciphering. The semantic web is a system of web resources (web pages) that are both human readable but also machine readable, ie. they present information in a very structured way that even computers can comprehend. The semantic web has many layers beginning with URIs -> RDF -> RDFS -> OWL. For a quick description of the relationships between the layers see http://stackoverflow.com/questions/1740341/what-is-the-difference-between-rdf-and-owl
URI - Uniform Resource Identifier. A string of characters used to unambiguously identify a resource. Typically URI's participate as parts of a fundamental data structure called a triple. A triple is a way of identifying an object/resource and assigning a value to it or a relationship to another object/resource. See http://en.wikipedia.org/wiki/Uniform_resource_identifier
RDF - Resource Description Framework. RDF is a common data interchange standard for the semantic web. It provides a way for networked data stores to share information and even to make inferences about the data. RDF allows data to be shared on the web without extensive collaboration between data providers. In this way RDF documents contribute to the network of linked open data that make up the growing semantic web. See http://en.wikipedia.org/wiki/Resource_Description_Framework.
OWL - Web Ontology Language. OWL like RDF is part of the stack of technologies used for the sementic web. OWL is a language for encoding ontologies, i.e. it is a way of formally encoding rich descriptions of knowledge in a domain such that computers can understand data and make inferences about the data.