The Arreat Summit is a sub-website that is part of the Battle.Net website. It is loaded with invaluable information, making it an essential resource for players.

The most common use for Arreat Summit is for its runeword list. It has a list of every single runeword, exactly how to make them (What items and sockets are required, etc) and pictures of exactly what they do.

I have also found its Weapons and Armor section very valuable as well. Here, you can look up any base item and learn how many sockets it can get, what its range is, what its requirements are, what its speed is based on class, and more.

Learn about:

-Sets & Uniques
You can use its set item section to know what bonuses you will get for sets and, of course, there is a unique items section.

-Runes
There is a runes section that you can use to determine what runes combine into what other runes.

-The Cube & Crafting:
There are sections for Horadric Cube recipes and crafting.

You can use this site to learn technical things too, like the affixes and prefixes that magic/rare items can have that determine the modifiers they get.

To summarize, Arreat Summit is amazing. The only information I don't go to it for is Breakpoints. (For some odd reason it doesn't have a section for that).