A physical standby is a block-for-block copy of the primary using Redo Apply with different levels of protection against data loss, depending on desired performance and resource restrictions. A snapshot standby is almost identical to a standard physical standby except that it is a snapshot of the data at a point in time. This type of database was designed for performing various testing scenarios without affecting the production database.
A logical standby is a read-only copy of the primary (production database) where redo data from the primary is applied while the database is open, via a process called SQL Apply. Additional read/write schemas can be added to the logical standby while still protecting the primary information in read-only mode. Certain data types are not supported and some DDL is skipped during the SQL Apply process. Logical standbys can also be used for Rolling Upgrades, which is purported to be the smallest amount of downtime achievable...