PostgreSQL capabilities

http://networkdna.com/database/index.html mentions that PostgreSQL is
capable of "Online backup". What does that exactly mean?
I'd like to be able to run a synchronization (in MS Access terms) (or
"incremental backup"???), i.e. to have two databases in two locations,
normally using only one of them and updating the other one. (Normal full
backup looks unrealistic for 300MB over 1/3 of a T1.) In case the
communication line between the two centers fails, users at the two ends
should be able to use the local databases, and changes made during
communication downtime be appended to the other database after communication
resume.

Thanks,

Mihai Gheorghiu