Sitecore is a leading .Net based web content management system (CMS) strongly focused on customer intelligence.
http://www.sitecore.net/
CMS
Content Management System: Set of tools that supports the creating, managing, and publishing online or offline (printed) content.
Content Personalization
Changing the look & features of the page depending on visitor.
XP
Sitecore Experience Platform: digital marketing platform, containing Sitecore XM, xDB, Sitecore Forms and Marketing Automation.
XM
Sitecore Experience Management: web content management system, multisite and multilingual.
xDB
Sitecore Experience Database: collects all page visitors interactions from all channels in a central, big data repository.
xConnect
xConnect is the service layer introduced in Sitecore 9, which provides read, write and search of xDB data, for any trusted client or device.
MongoDB
Key xDB component, data storage for interactions created by page visitor.
https://www.mongodb.com/download-center. From Sitecore 9 it can be replaced with Ms SQL 2016, or other no-sql database.
Experience Platform
Combination of Sitecore CMS, xDB, and other experience marketing applications
Content Editor
Standard Sitecore tool used for content editing. Content in content editor is displayed in form of the expandable tree.
Experience Editor
WYSIWYG (what you see is what you get) editor, where content editors can edit & preview whole pages.
Master
One of main Ms SQL databases in Sitecore, storing content managed by content editors
Web
One of main Ms SQL databases in Sitecore, storing published content visible by website visitor
Core
One of main Ms SQL databases in Sitecore, storing users and several platform settings.
CD
Content Delivery: Server hosting website version available to external vistors, typically it uses Web database for content items.
CM
Content Management: Server hosting website version available for Content Editors, typically it uses Master database for content items.
WFFM
Web Form For Marketers: additional component for forms creation and defining actions after forms submit. In version 9, Sitecore introduced new module, which will replace WFFM in the future: Sitecore Forms.
EXM
Email Experience Manager: built-in tool for mass mailing with similar features as Sitecore content management for websites.
FXM
Federated Experience Manager: additional component used to add some of Experience Platform features to external website.
JSS
Sitecore JavaScript Services: SDK for JavaScript developers allowing to access Sitecore data from modern JS frameworks like React, or Angular.
SPEAK
Sitecore Process Enablement & Accelerator Kit: framework for extending Sitecore application. Starting from version 3, SPEAK is based on AngularJS