Hello Roger,
The links below may help to answer your questions. In a nutshell, the difference is the frequency at which the volume is being replicated. Synchronous is continuous to ensure no data loss between the volume copies and used for high-bandwidth, low-latency links. Asynchronous replication allows more leniency at the risk of some data loss. However, it is better for higher-latency links so you still have an option to use Storage Replica.
Replication occurs in one direction and the destination should not be brought online to avoid corruption (except for testing with 'Test Failover')
If you want to run VMs on both sites and still wanted to use Storage Replica, you could set up a source volume at each site and have it replicate to the other.
Best regards,
Trent