You can also use Zotero to generate citations without downloading a plugin for your word processor. Select an item from the center column of Zotero Standalone and drag it into the open document of your word processor, email, blog post, etc. Zotero will automatically create a formatted citation for you.
To add several citations at once, highlight the items you'd like to cite (use control-click to select items in your library that are not listed next to each other), then right-click and select "Create Bibliography from Items..."
Zotero's citation plugin allows you to insert references from your Zotero library into papers that you write and generate a bibliography. Depending on whether you are using Zotero Standalone or Zotero for Firefox, the way you install word processor plugins will differ.
Zotero's Citation Style Repository has over 8,100 citation styles in it, all of which were written with the open source Citation Style Language (CSL). You are welcome to use it to edit citation styles or to add a new citation style to the repository.