Each item will have attributes, which are similar in concept to a column in a spreadsheet or a column in a database table. Each attribute is a key/value pair. The key is the unique name for the attribute and the value is the textual data for that key. SimpleDB is schemaless and allows you to have different attributes for each item in a domain. This is impossible in a relational database world where you must define your table schemas up front, and every time you need to add a new field or column, you must upgrade the schema for the database, or your existing applications might start throwing errors. SimpleDB frees you from this upgrade and maintenance cycle, and gives you the freedom to use this flexibility to your advantage when designing your applications.
If you add a new attribute to an item in a domain, only that item will have that attribute, and all the other existing items in the domain will hum along nicely without that additional attribute! You can see this in the following...