Therefore! I've put together a list of indispensable e-book/Internet-related terms I think you should know. If you think of any more (and I'm sure you will), please don't hesitate to post them in the comments.
CSS (Cascading Style Sheets). A system for separating a web page's or e-book's style/formatting from its content. For example: rather than putting a tag around every block of text that specifies the font as Garamond, you can just have CSS declare that all text should be in Garamond from the outset.
Think of it as like giving directions from the passenger seat of the car: you can just tell the driver, "go straight until I say otherwise" from the outset, rather than saying, "keep going straight" at each intersection.
E-book (also ebook, eBook). An electronic book available in a wide variety of formats (e.g. AZW, EPUB, MOBI, PDF) on a variety of devices (e.g. Amazon's Kindle, Barnes & Noble's Nook).
EPUB (also ePub, ePUB, EPub, epub). The industry standard e-book format. It's basically a zipped-up website.
HTML (HyperText Markup Language). The language used to write websites and e-books. It's currently on version five (HTML5).
PDF (also .pdf). Standing for "Portable Document Format," a .pdf is a file format readable by many (but not all) e-reading devices. Its primary selling point is that it represents documents independent of the machine it runs on, so a .pdf e-book looks the same no matter what devices is used to read it. For this reason, however, .pdf files are not reflowable (see below).
Reflowable content. Content (words, diagrams, illustrations, &c) that can change or "reflow" depending on the device designed to read it. Text "reflows" when you change the font size on your Kindle or when you switch back and forth between devices with different display sizes.
This is one reason e-versions of the same title look different on different devices; another is that different e-tailers do different things to the source files they receive from publishers before making the book available to the consumer.
SEO (Search Engine Optimization). Basically, this is the idea of improving your visibility via search engines on the Internet. For example: if you Google "[your name] author," you want your personal website to be one of the first few hits. Taking into account how search engines work and what search terms people use, it's possible to move up the list of results (often dramatically).
XHTML (eXtensible HyperText Markup Language). A family of XML languages (see below) that serves as an alternative to HTML (above).
XML (eXtensible Markup Language). Wikipedia says it best: "A set of rules for encoding documents in machine-readable form." If you're using Microsoft Office 2007 or later, you're already familiar with one of XML's many uses (it's the "x" in ".docx," ".xlsx," &c).