What's the difference between a repository item and an item descriptor in ATG repositories?
A repository item contains the data for one record in your repository, which means that you use repository items to access the data that is stored in your repository and (usually) your database.
An item descriptor defines a named item TYPE, so this is more about the structure of the data. Each repository item always has a defined type that again defines the list of available property names for the item, the data types of these properties, etc... - and this data type definition is the item descriptor.
If one tried to translate these two concepts to the world of relational databases, a repository item would be something like one row (one record) of data that is stored in the database. An item descriptor would be something like a table.