Here is RiotGTez with details on custom item sets:
"The item shop has been your one-stop spot for Infinity Edges, Ohmwreckers and Doran's enchanting accessories for quite some time. Happily, you'll now have a second, in-client place to research and plan builds. The new Item Set tab lists every in-game item and lets you customize your builds for your favorite setups.
Once in the client, click on your profile and find the new Item Sets tab we've stuck in there for you. Inside the tab, you'll see three columns.
The left-hand column is where you generate new item sets and give them individual names. The second column lets you sort items exactly like you would in the in-game item shop. To create your custom item builds, you'll drag and drop an item into a block in the third column.
"What's a block?" you say. The recommended build that you see in-game is made of blocks. They're called things like "Starting Items," "Essential Items," etc. Your blocks can be re-named to whatever you want.
So you've created your top-secret builds. Now it's time to assign them to specific champs and maps. There are two tabs at the top of the third column. You created your item sets in the 'Items' tab; switch it over to 'Maps and Champions' to associate your new build with as many maps and champs as you like.
Figured out a hyper AP build for Ziggs, Yi and Zilean on Howling Abyss? Sweet. Want a tanky jungle build for Sejuani, Nautilus and Amumu on Summoner's Rift? Do it up. It's your call.
Just remember to tap that save button and your custom build will be ready for quick in-game clickery. You'll spend less time in the item shop and more time helping your team secure the W.
This was also followed by a brief FAQ on the new system:
"What are item sets?
Item sets are a way for you to create new recommended item pages that will show up in the item shop when you play.
How do I make item sets?
Select the item sets tab in your profile. Add a new page and start dragging items into it. You can create new blocks so that you can organize your items by offensive, defensive, jungle, or whatever you think of. When you save your page it will be available in-game through the item shop.
Does this save to my account?
Currently, item sets will only save to your local machine, but we want to get server-side saving to you soon. We know that this will be much better for players when they can use the feature across multiple computers, and we're working to deliver that. The server-side version will take some time since it requires additional platform work, and we want to make sure we don't impact service stability.
How does this work with community-made recommended item builders?
Recommended item pages created using 3rd party tools should still work. They can co-exist with the ones created using the item set feature, but you will not be able to use the Item Sets feature to edit your 3rd party sets.
Can I have more than one item set per champion?
Yes, you are able to select as many champions as you'd like to be associated with an item set. You can also opt to make your item set available for all champions if you prefer.
What does this look like in game?
The recommended tab now has a drop down. Custom item sets directly associated with the Champion you are playing will appear first in the drop down, followed by recommended items, followed by more general item sets and sets that are associated with multiple champions and maps."