Generally available since 2015, Amazon Aurora is built on a proprietary distributed storage engine that automatically replicates 6 copies of data across 3 availability zones for high availability. From an API standpoint, Aurora is wire compatible with both PostgreSQL and MySQL. As described in "Amazon Aurora under the hood: quorums and correlated failure", Aurora uses a quorum write approach based on 6 replicas. This allows for significantly better availability and durability than traditional master-slave replication.
Horizontal write scalability
By default, Aurora runs in a single-master configuration where only a single node can process write requests and all other nodes are read replicas. If the writer node becomes unavailable, a failover mechanism promotes one of the read-only nodes to be the new writer.
Multi-master configuration is a recent addition to Aurora MySQL (not yet available on Aurora PostgreSQL) for scaling writes that involves a second writer node. However, because all of the data is now present in both the nodes, concurrent writes to the same data on the two nodes can lead to write conflicts and deadlock errors that the application has to handle. A long list of limitations include the inability to scale beyond the original two writer nodes as well as lack of geo-distributed writes across multiple regions.
Multi-region active/active deployments with global consistency
Geo-distributed deployments in Aurora essentially involve a single primary region for handling writes and multiple other regions as read replicas. So globally-consistent multi-region deployments are not possible in Aurora.
Fully open source
Aurora is Amazon's proprietary database that does not allow users to build cloud-neutral applications and run on cloud-neutral orchestration technologies like Kubernetes.
Relevant blog posts
The following posts cover some more details around how YugabyteDB differs from Amazon Aurora.