Biomass emissions are considered carbon neutral. That happens because the carbon contained in biomass would end up in the atmosphere either way and forms part of the carbon cycle.
Having said that you also have to factor in any potential life cycle emissions. Your don't necessarily use biomass as is, but you usually convert it to something else first - for example sugar to ethanol, fat to biodiesel, woody biomass or energy crops to pellets, chips or briquettes. All of these process require energy inputs, material inputs etc. As such, under a life cycle analysis perspective, biomass does have carbon emissions - although that can be said with every energy system out there.
Remember that everything we do will have an effect on the environment. It's up to us to decide how much of an effect we are willing to accept.