SELECT db1!transaction.item, db2!inventory.description JOIN db1!transaction.item=db2!inventory.item FROM db1!transaction ...

SELECT transaction.item, inventory.description
FROM db1..transaction transaction, db2..inventory.item inventory
where transaction.item=inventory.item

If the databases are in different servers/boxes you will not be able to run this query
db1..transaction can also be written as db1.dbo.transaction
db1..transaction ->transaction is the alias name