RSpec is a testing tool for Ruby, created for behavior-driven development (BDD). It is the most frequently used testing library for Ruby in production applications. Even though it has a very rich and powerful DSL (domain-specific language), at its core it is a simple tool which you can start using rather quickly
Capybara is a web-based automation framework used for creating functional tests that simulate how users would interact with your application. Today, there are many alternatives for web-based automation tools, such as Selenium, Watir, Capybara, etc. All of these tools have the same purpose, but there are slight differences that make each of them more or less suitable.
The main characteristic that developers are aiming for is the ability to have tests that are modular, easy to write, and easy to maintain. This is especially true in Agile/TDD environments where writing tests is second nature. These tests are expected to give good and fast feedback on code quality. As time goes by, the number of tests grows and it can be a real nightmare to maintain the tests, especially when the tests are not modular and simple enough