PulseAudio is a sound server (also does mixing, processing, etc.)
It does not directly interface with the hardware (sound card). In fact, it usually uses ALSA to talk to the sound card.
So it's normal for components of both PulseAudio and ALSA to be installed at the same time, and this is the default in Ubuntu. With the default installation, no, they do not conflict.
In general, PulseAudio is preferable to using ALSA to provide the "sound server" (software-level sound support); however, professional audio users often prefer using JACK instead of PulseAudio for lower latencies.