The organic material obtained from the plants is called as biomass. The plants absorb energy from the sun by a process called photosynthesis and this is stored within the plants. The animals and humans beings eat plants, hence the energy absorbed by the plants get stored within them also.
We do not consider the living plants as the biomass. The garbage left over by the plants in the form of dead trees, broken branches of the trees, the wasted crops, fallen leaves, wood chips etc are considered as the biomass. You can also consider bark, saw dust from the lumber mills, livestock manure etc also as the biomass.
The other garbage and waste materials that are directly and indirectly made from the plants can also be considered to be the biomass materials. For instance the paper is made from the plants. Even in this age of computers tons paper goes wasted everyday in private and government organizations. Lots of paper is also wasted after personal use by people every day. The wasted paper is also considered to be the biomass.