UGC stands for user-generated content and as the name implies It is content that is produced or created by people like you and me and is shared across many platforms. These may be any form of media such as blogs, videos, pictures, music, podcasts, articles, etc and that is created using a online service or plattform on the web; e.g youtube, behance, wordpress, etc.
Sites like OpenStreetMap for example encourages people to contribute on their site, In fact all their content depends on people willing to track and share locations on their site. Therefore it is a service that is free to use and is powered by the community for the community.
Other sites like wordpress include a plugin you can add to your site to manage your posts, images etc. It essentially gives you the choice to change and monitor progress of both code and well, themes you may choose for your site if you so happen to want it this way. They have a nice built-in native UI that is cleanly designed and easy to follow. Ultimately wordpress is open source and anyone can come and change and even rewrite the code for the plugin if so they desired in order to make something work a specific way, although if you could indeed do that then using such plugin would be unnecessary in the first place so yh… maybe no.
There’s also sites like tumblr (open source) for example which I am currently using to host a personal startup project with some friends. I’ll leave the url here just in case: http://monsterhut.net/. We got a plugin from tumblr and added it to our server, as a result we ended up having the choice to edit our site using a pre-made UI that comes built-in the plugin, this gave us a lot of themes and options to choose from etc. We and by that I mean a friend and I edited the code to better fit our idea and I made a logo for the brand as well as a youtube page to inform people of who we are as well as our ideas and well just general stuff like gaming sessions etc. This all means that we both generate (produce) and consume content, we are ‘prosumers’, and this all relates back to UGC! we got a platform (tumblr) to manage and create content then after making it we shared it back for other people to enjoy :D.