Yeah well, even thou wikipedia have it all I feel sometimes that short explanation can do good. So here goes the first of a serie on terminology.
XPATH : it is a language used to travel through a XML document. Yes, you are using it when you use CSS styling because that is the language that enables you to find, for example, the first of a node.
ul li:first //shall find the first li element of each ul
You can get much more information here.
ETL : Stands for Extraction Transformation and Loading. This term comes mainly from the Business Intelligence field of expertise. Generally we would talk more about ETL tools. Those tools can extract data from one source, transform it and load it into a target database or whatever can old data. We are talking of data integration.
The main companies providing these kind of tools are : Informatica, SAP, Oracle, …and you also have open source ETL tools like: Talend, Jasper, Pentaho, …
CDN : Content Delivery Network. This is a technique used to easy the work of a webserver by pointing some assets of a loading page to other servers. By doing that, the page loads faster because the server doesn’t have to serve those files. Often used for libraries, like jquery for example.
That’s it for now. I new term a day keeps the ignorance away 🙂 (you got three here)