The Closure Compiler compiles JavaScript into compact, high-performance code. The compiler removes dead code and rewrites and minimizes what's left so that it downloads and runs quickly. It also checks syntax, variable references, and types, and warns about common JavaScript pitfalls. These checks and optimizations help you write apps that are less buggy and easier to maintain.
The Closure Library was a broad JavaScript library, but is now in maintenance mode. Other more targeted solutions should be preferred.
The Closure Linter enforces the guidelines set by the Google JavaScript Style Guide. The linter handles style issues so that you can focus on coding.
Closure Stylesheets is a system that adds a number of Google extensions to the standard CSS language. With these extensions, you can define and use variables, functions, conditionals, and mixins in your stylesheet, making your stylesheets more readable and maintainable. An included tool can compile the stylesheet down into standard CSS and supports minification, linting, directionality (right-to-left flipping), and class renaming.

Closure Templates simplify the task of dynamically generating HTML. They have a simple syntax that is natural for programmers. In contrast to traditional templating systems, in which you use one big template per page, you can think of Closure Templates as small components that you compose to form your user interface.

Closure Templates are implemented for both JavaScript and Java, so that you can use the same templates on both the server and client side. For the client side, Closure Templates are precompiled into efficient JavaScript.