Ui Patterns help in answering questions like:
- When do you use a side menu instead of a top menu?
- When do you use a fly out menu instead of an icon menu?
- Do I really need a news ticker?
- I like accordion navigation, but are they necessary for my site

A UI pattern, just like a coding pattern, is a description of an optimal solution to a common problem within a specific context.

A pattern has four components
a title
a problem
a context
a solution


Example?
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We use the concept of patterns everyday in the real world. EG. Here's a pattern concerning eating:

Title :
Refridgerator Cooling

Problem :
Food is too hot to eat

Context (or when to use this) :
When the eater is impatient, but has access to a refridgerator, and nepa is behaving.

Solution :
Place your food in a plate, and put it in the refridgerator, towards the top shelf so as to have minimal impact on other food in the fridge, and so it will not cool at a fast rate.

And there you have it, I've just written a pattern called 'Cooling' Silly example, but a simple concept, right?


So?
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If you've continued reading up to this point, here's why it was worth it: there are dozens of proven UI patterns for interaction design.
Martijn van Welie, (Ph.D. Human Interaction design), is a long time contributor to the topic of UI patterns, and is respected by designers worldwide, including many experience designers at Adobe. The best part, his findings are publicly available on his website.

Have look, and enjoy:
http://www.welie.com/patterns/index.php

Also, see Yahoo's design pattern library:
http://developer.yahoo.com/ypatterns/index.php

Hopefully some of you will understand and apply what you learn, and watch, over time, how much more effective your websites become.

Update: Found even more additional design patterns and interface guidelines here:
http://www.cpluv.com/www/feeditem/5046

Enjoy