Hello Tiago,
First of all, welcome to OutSystems and to the Community :)
In OutSystems, you have 3 types of applications: Traditional Web, Reactive Web and Mobile.
Each of them has its own programming concept. Traditional Web Applications run only server-side while Reactive Web and Mobile Applications run client-side and server-side.
Responding to your question, both OutSystemsUI and OutSystemsUIWeb are libraries containing several UI widgets / patterns that you may use in your applications.
The main difference between them is that OutSystemsUI is designed for Reactive Web and Mobile Applications:

While OutSystemsUIWeb is designed for Traditional Web Applications:

Hope that this helps you!
Kind regards,
Rui Barradas