Bootstrap is a design framework, for building easy, beautiful looking components and responsive design. It also has basic js components that makes life of any developer easier, prominently of a backend developer, who is not very experienced with HTML design rules.
So, just put the css, check the examples, and replicate it in your project. That’s the major flow with Bootstrap.
Angular, is a JS framework, for building complex web apps. It comes in line with Backbone, Ember Js.
Take a simple example of a form, where you wish to make an ajax call on Submit. Now you can make the form, beautiful, by Bootstrap CSS, but you need to make use of javascript or in this case , angular, to make the ajax call and do the required functionality.
These go hand in hand in web apps, they are not competitors.