If you're a ruby developer, plugging a gem into a project is second nature. Most projects now use bundler to manage gem dependencies and versions, as it is so effective at doing so. If you've been doing this for long enough, then you will have come across the pessimistic version constraint operator, at least unknowingly.
Git submodules are a way of linking to a repository inside another. For example, if your project needs to use someone else's project that's hosted on Github then you can use it as a submodule rather than including all the code from that project. This has the added benefit of being able to track and include changes from the submodule repo, rather than having to update project files yourself.
One of the tools I use on a daily basis when browsing the Internet is the developer tools that come with webkit based browsers (i.e. Google Chrome and Safari). The "Elements" tab gives you a slick breakdown of the entire page DOM, and allows for easy manipulation.