Collections are groups of themed items which you can combine into special, more valuable items.
When you purchase an item that's part of a collection, you'll receive a notification in the top=right. Click the Add button or click the collections button along the bottom to see your collections.
Items are not part of a collection until they're added. Click the Add button to add and item to a collection. Items that are part of a collection aren't stored in your inventory, but you can remove them at any time.
Once you have all the items, click Complete Collection to transform the items into the more valuable Collection time. You can now sell that item for massive profits or keep it in your inventory to collect.
You can complete collections over and over again. Rake in the cash!