When you're writing a post, referencing and quoting external material is a great way to back up your posts, provide sources, and increase the quality of what you're writing. There are, however, a few thing to keep in mind when quoting external material.

Referencing online material

When you're referencing or quoting material that can be found online (such as Wikipedia or similar), please make sure to include a link.
Pay attention to what license the content is published under - for instance, Wikipedia licenses text under the Creative Commons Attribution-ShareAlike 3.0 license, which requires attribution and a link back to the original source.

Referencing offline material

When quoting from offline material, such as a book, please make sure to always include at a very minimum the title and author of the work. Including a chapter number or page number when applicable is ideal.
Be sure to only quote whatever text is relevant. While quoting for knowlege sharing usually falls under Fair Use rules, which allows the use of copyrighted material, limiting the sharing of copyrighted works to only what's necessary for your post is in general a good idea.

In any case, whether your original source is online or offline, please clearly mark the quoted material as being quoted from somewhere and is not your original content. This is done simplest with blockquote formatting (see the formatting help for help with formatting).