HTML is the structure of the webpage - the headers, articles, etc.
CSS styles the HTML and positions the elements more precisely.
All three of them are parts of a whole to make webpages.
Bootstrap makes front-end web development faster and easier. It's open source.
You don't have to write everything in CSS or js. You have to just pick a theme/button/Table/Forms/etc which you like and you can copy and paste into your code.
Cascading Style Sheet and is used by web pages to help keep information in the proper display format. CSS files can help define font, size, color, spacing, border and location of HTML information on a web page.
HTML - The skeleton of a webpage, HTML only allows you to make a webpage without any proper style or layout.
CSS - CSS is like skin of a skeleton, CSS allows you to give proper layout and style to your HTML webpage.