1. 1. Syntax: Chai is a JavaScript assertion library that provides a fluent interface for making assertions in tests, while Cypress is an end-to-end testing framework that uses Mocha and Chai under the hood. Chai uses a more traditional syntax with explicit assertion keywords like expect, should, and assert, whereas Cypress uses a more declarative syntax with built-in commands like cy.get and cy.contains to interact with elements on the page and make assertions.
2. 2. Execution Environment: Chai can be used in any JavaScript runtime environment such as Node.js, browser, or even mobile applications, whereas Cypress is specifically designed to run in a browser and requires a separate browser window for running tests.
3. 3. Testing Scope: Chai is primarily used for unit testing where tests are written to validate small, isolated units of code, while Cypress is designed for end-to-end testing where tests simulate real user interactions with the application and cover multiple components and functionalities.
4. 4. Test Runner: Chai relies on external test runners like Mocha or Jest to execute the test cases, whereas Cypress has its own built-in test runner that provides a visual interface to interact with the application under test and see the test results in real-time.
5. 5. Time Travel: Cypress has a unique feature called "time travel" that allows developers to debug their tests by stepping through each command and inspecting the application's state at any given point, which is not available in Chai.
6. 6. Automated Browser Tasks: Cypress provides additional capabilities for automating browser tasks like refreshing the page, clearing cookies, handling network requests, and more, whereas Chai is primarily focused on making assertions and does not have built-in support for browser automation.

In Summary, Chai and Cypress differ in their syntax, execution environment, testing scope, test runner, time travel feature, and support for automated browser tasks.