Lightning Web Components (LWC) from Salesforce are based on standard Web Components built using HTML and JavaScript. They are lightweight, easy to build, and perform well in modern browsers. When building LWCs, you’ll become familiar with the concept of composition: piecing together simple building-block components within the body of a more complex component.
Regarding composition, LWC leverages the Shadow Document Object Model (DOM) web standard, which encapsulates the internal structure of a Web Component and makes it inaccessible to code and components outside of the component. The alternative to this approach is Light DOM, which Salesforce makes available in beta.