3.03 is a major update. See the Change Log tab for more details.
LootLink is a browseable, searchable database of every item that you come across in game. The database is initially empty, but as you encounter and interact with item links, drops, quest items, vendors, the auction house, and other players' items, they are added to the database. In practice, I find that it doesn't take very long to build up a large collection of items.
It's then easy to search for interesting things in game. Want to find non-combat pets? Search for All text:summon and dismiss. How about epic leather armor with plus healing? Try: All text:healing, Rarity:Epic, Type:Armor, Armor subtype:Leather.
For users familiar with Lua's regular expression syntax, you can use a regular expression for the All text field if you hold ctrl while clicking Okay in the search box. If you want to find epic leather armor with at least +100 healing, you can modify the above example by using All text:healing done by up to 1.. and and holding ctrl while choosing Okay.
From the browser, you can shift-click items to add a link to that item into your chat edit box or ctrl-click them to try them on in the dressing room.
Display the browser by binding a key to it -- look for the LootLink Buttons keybinding section, which contains the Toggle LootLink binding -- or by typing /lootlink or /ll in a chat edit box. There are various other options accessible via chat commands; use /ll help for a list.
New in 3.03.3, LootLink supports chat auto completion of links. Just start typing [ and any part of an item name to get a popup window with completion matches. Cycle through the matches with Tab and use ] to select your choice, which will replace the fragment that you've typed with the link to the selected item. See the Change Log tab for more details, if interested.
If you'd prefer to turn this functionality off, use /ll autocomplete to toggle it off or on.