UDR (Uni-Directional Replication for PostgreSQL) is a cut-down version of BDR that will run on an unmodified version of PostgreSQL 9.4. It is BDR with all the features that can't work on unmodified PostgreSQL 9.4 removed.
Most importantly UDR can only replicate in one direction, so it doesn't support multi-master. It cannot replicate DDL (schema changes), so users must manually synchronise database schema changes. Unlike BDR, there are no row-conflict resolution features in UDR.
UDR is mainly useful for read-replicas and for performing minimal-downtime upgrades to new PostgreSQL versions (or to PostgreSQL 9.4 with BDR). It offers some significant advantages over streaming replication and over trigger based solutions; see Differences between logical and physical replication and Differences between BDR and trigger-based replication.
You can get UDR from the same download locations as BDR (and in fact it's the same source code, just with different options).