Brian Swan (of Microsoft) has a new post to his blog today looking at using the Windows Azure Table Storage from inside of your PHP application.
An overly-simplified answer: Windows Azure Table Storage is structured storage in the cloud provided by the Windows Azure platform. It allows you to create any number of tables, which can each contain any number of entities. Each entity can in turn contain any number of properties. A picture is worth a thousand words here.
Think of it as almost a Microsoft-created version of a NoSQL database engine. He shows you how to get started by making an Azure account and setting up a new project for you to work from. Then, using the Azure SDK, he shows how to easily make a connection and do all of the usual CRUD (create, read, update, delete) operations on this new project.