They're technical terms for "blocking input/output" and "non-blocking input/output". The next release revises the settings to make more sense to people that aren't us.

In a nutshell, blocking i/o uses more resources and memory, but has been in Java for longer and might have less unexpected problems. Non-blocking i/o is much faster and uses less resources and memory, but is relatively new to Java (and LimeWire) and could contain some problems we weren't expecting.

DAAP is going to be a good load-test for ultimately converting the rest of LimeWire to NIO.