BDR/UDR use PostgreSQL's logical decoding feature to implement a low overhead logical replication solution. It has significant advantages - and some disadvantages - when compared to PostgreSQL's older physical (block-based) streaming or archive-based replication with warm or hot standby
Logical replication has a different set of trade-offs to physical block-based replication. It isn't clearly better or worse. Physical replication is a lot simpler, has less lag for big transactions, is supported by older versions and may require less disk I/O, but generally consumes more network bandwidth, can't replicate a subset of databases or tables, and can't support multi-master or cross-version/cross-arch replication. Which solution you should use depends on what you need to do.
The major differences between physical replication and logical replication as implemented by BDR/UDR are:
Multi-master replication is possible (BDR only). All members are writable nodes that replicate changes.
Data from index writes, VACUUM, hint bits, etc are not sent over the network, so bandwidth requirements may be reduced - especially when compared to physical replication with full_page_writes.
There is no need to use hot_standby_feedback or to cancel long running queries on hot standbys, so there aren't any "cancelling statement due to conflict with recovery" errors.
Temporary tables may be used on replicas.
Tables that aren't being replicated from elsewhere may be written to (UDR and BDR).
Replication across major versions (e.g. 9.4 to 9.5) is supported.
Replication across architectures and OSes (e.g. PPC64 Linux to x86_64 OS X) is supported.
Replication is per-database (or finer grained), whereas physical replication can and must replicate all databases.
BDR's logical replication implementation imposes some restrictions on supported DDL (see: DDL replication) that do not apply for physical replication
Commands that affect all databases, like ALTER SYSTEM or CREATE ROLE are not replicated by BDR/UDR and must be managed by the administrator.
Disk random I/O requirements and flush frequency may be higher than for physical replication.
Only completed transactions are replicated. Big transactions may have longer replication delays because replication doesn't start until the transaction completes. Aborted transactions' writes are never replicated at all.
Logical replication requires at least PostgreSQL 9.4.
Logical replication cannot be used for point-in-time recovery (though it can support a replication delay).
Logical replication only works via streaming, not WAL file archiving, and requires the use of a replication slot.
Cascading replication is not (yet) supported by logical replication.
Most users will want to use physical replication and/or WAL archiving for redundancy, high availability, backup and PITR. Logical replication is well suited to data integration, data movement and data aggregation (often as an alternative to or combined with ETL), for scale-out and for distributed multi-master deployments.
It's possible to replicate between different PostgreSQL versions, operating systems and/or processor architectures using logical replication because it can fall back to sending data in text form - just like SQL. Where the servers are compatible it can use the faster-to-process binary representation or an intermediate form. Logical replication cannot prevent all possible incompatibilities though - for example, it isn't possible to replicate a type added in PostgreSQL 9.5 to PostgreSQL 9.4 because 9.4 has no way to store and represent it.
Unlike physical replication, which replicates all databases on a PostgreSQL install, logical decoding permits (and requires) separate replication of each database. It can also replicate a subset of tables within a database. It is not possible to configure wildcard replication of all databases on a server in logical replication. You can replicate multiple databases, but each database must be configured separately.
Temporary tables may always be created on all nodes, even if they are also receiving replicated data. There's no prohibition against doing so like it exists for PostgreSQL's block-level replication features.
Local writes are not limited to temporary tables. UNLOGGED tables may be created even on nodes that are receiving changes from upstream/peer nodes. Additionally, replication sets allow changes to only a subset of tables to be replicated if desired, so some normal tables may be excluded from replication. This makes BDR/UDR very useful for use cases where significant work is done on nodes that also receive replicated data from elsewhere.
Logical replication doesn't start replicating a transaction until it commits. This can cause longer replication delays for big transactions than physical replication, where the transaction's changes get replicated as soon as they're written. It also lets logical replication entirely skip replication of writes by aborted transactions. Future enhancements to logical decoding may permit streaming of transactions before they're committed.
Because logical replication is only supported in streaming mode (rather than WAL archiving) it isn't suitable for point-in-time recovery. Logical replication may be used in conjunction with streaming physical replication and/or PITR, though; it is not necessary to choose one or the other.