Well React and Angular have nothing to do with Web Components. But first things first.
Web Components are a new technology where you are building a complete seperated component of of something. For example an image slider. You have the markup, css and js seperated into this one compontent you are building. Which is a bit confusing at the beginning but in the end its pretty cool.
Because you can reuse your compontents that you've build for another project.
You are making your own html fake tags, for using the components. For example if you making your slider compontent you can use something like <slider></slider> and in your webcompontent you have something like
<div class="main-slider">
<div class="container">
<img src=...>
<img src=...>
<img src=...>
</div>
</div>And the js for the webcomponents will see your slider tag, and create the content of it into the shadow dom.
So you can basically build very clean markup which gets rendered later into something more complex. it is pretty cool, because of the seperation of components it's easier to make changes. You exactly knwo which component to change and you can be sure that this changes will effect only this one component.
And the difference between ReactJs and AngularJs... well that are two different things. AngularJs is a full MVW frontend framework. You can make interactive interfaces and views. It has two way-data binding so whenever you change something in the view the model gets changed too.
ReactJS however is a frontend framework, too. But only for the View Layer of your application. It's pretty cool, too. Because in react you are also building your components which can require other components and so on.