Web Basics

• Client (Web Browser) and server (Web Server) usually talks using HTTP over TCP. UDP-based RTP is better for streaming.
• Web Browser can read HTML, XHTML and can show pages written in these languages.
• Cascaded Style Sheets (CSS) help separate design (color, font, etc.) from contents in HTML, XHTML, or XML. CSS has three variants: inline, internal, and external. Inline overrides Internal and Internal overrides External, hence cascaded. The cascading allows controlling individual elements (inline) or page (internal) while having a global control (external).
• XML is mainly used for data structuring and all tags are user-defined.
• Extensible Stylesheet Language Transformations (XSLT) helps transform XML into Browser-friendly formats.
• Client-side scripts or "embedded scripts" help make otherwise static pages, dynamic or interactive. They are kind-of like applets.
• JavaScript and VBScript are the most popular client-side scripts. 
• Cookie is kind of cache-info put into my machine by a site (through client-side scripts) to which I visited previously. It has a name, value, and expiry date.  
• Emails are sent to SMTP server and received from POP3 server. 
• Active Server Pages (ASP), PHP (originally Personal Home Page, nowadays PHP: Hypertext Preprocessor), and Practical Extraction and Reporting Language (PERL) are popular server-side scripting languages. 
• Common Gateway Interface (CGI): A script or executable program is a CGI script if it is inside of and executable by the server, triggered by the browser, and the result can be displayed on the browser. So, CGI scripts or programs can be written in C/C++, PERL, PHP, and ASP.