UGC:
User-Generated Content.
Created by people (individual creators), not brands.

WHY UGC?

UGC is a part of marketing, and allows brands to connect with individuals to get their product in front a specific demographic. It’s similar to influencer marketing, but different in that the brand can utilize the content for their own accounts on social media for advertising purpose. Helps brands build authenticity and trust within their target market!

UGC includes a variety of opportunities for engagement, from developing quality copy to attract keywords and engagement, to photography and video.

Some examples of UGC are:
Product demos (Photo and Video content)
Product  Testimonials
Brand Partnerships (Shout outs on YouTube)
Product Photography (images)
Unboxing
Get Ready With Me (GRWM)
ASMR
Vlog – Day in My Life
What I Ordered vs. What I Got
Before & After
Voice Over

Where does it come from?

Employees, brand loyalists (those on your email subscriber list!), Customers.

What do they do?
Create brand assets for you to utilize.
Help reduce your overhead compared to working with traditional influencers or celebrities.
Increase your engagement, reach, and trust with the public.

Is UGC PR?
UGC would be considered a small aspect of public relations curated between the brand and an individual, usually the individual is already aware of the brand and is able to speak honestly on the quality of service provided.

Sources:
https://blog.hootsuite.com/user-generated-content-ugc/
https://www.prsa.org/about/all-about-pr