PulseAudio is focused on desktop and mobile audio needs. It doesn't try to address low latency usage, but does provide seamless device switching, network routing, global per-application volume control and lots more great stuff.
JACK is focused on the needs of pro-audio and music creation users. It offers the lowest possible latency, complete routing flexibility between applications and audio hardware, and all audio is always sample synchronized - apps don't run ahead or behind of others. It doesn't provide the smooth desktop experience that PulseAudio is aiming at.