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Moodle is what's commonly known as a Learning Management System (LMS) or Course Management System (CMS). Moodle itself stands for "Module Object Oriented Dynamic Learning Environment". Moodle is a system for supporting teaching and learning and can be used in online or face-to-face learning. Your course might be split into modules, each module covering a different topic. On the Moodle page for a module, you can find content that supports teaching and learning including:
the slides for each lecture/presentation
pre- and post-lecture reading material
a forum for discussion of topics covered in class
Note that these are the kinds of things you can find on Moodle, but that doesn't mean to say that you will find those things on every Moodle course. Your teachers will all use Moodle differently.
This might make Moodle sound very fancy and complicated, but it's not. Moodle is just another website and if you can find your way around websites, then you can use Moodle.
This is a short video created by Cork Training Centre (thanks to Brenda O'Connell for permission to use this) to gently introduce Moodle to their users but may be useful for everyone.
If your college or training centre has a guide to using Moodle, it's strongly suggested that you read it! If you can't find a guide, then have a look on the links page in this section for Moodle resources that might be helpful.