Most websites use style sheets to define how things will look (e.g. fonts, text size and color, borders, alignment and placement of elements, background colors, background images, etc.). For example in DU's main style sheet you can find the rule for excerpts (inserted in a post when you hit the excerpt button or type [div class="excerpt"] [/div] into your message):


A 'user style sheet' is a file that resides on your computer that will tell your browser that you want to substitute the website's style rule for a particular element with something you define yourself. Depending on the browser/platform implementing a user style sheet is very easy or virtual impossible.

Some of the things I am doing on DU with my user style sheet are: hiding signatures, adding a rounded border and drop-shadow to excerpt boxes, right justifying and hiding most of the edit text message and changing the text color to light grey, hiding the post count and profile/mail/journal/ignore icons by the username in posts....

ETA: I guess I should take a screenshot to show what posts look like when I browse DU. This is what my post (before adding this image) looked in my browser:

[div style="display:inline-block; border:8px solid #99bbff;"]