Biological molecules are complex molecules synthesised by living systems. Although small molecule drugs have been dominant, biologicals are becoming more common due to their improved specificity and the ability to design them.
Biological molecules are the simpler type of biological drug, and would include mRNA vaccines (such as the Covid-19 vaccines) or interfering RNA (RNAi, siRNA), amongst others.
Monoclonal antibodies are the most complex biological drug, having to be synthesised in living cells. This can be by using immortal cell lines, producing hybridoma cells. Monoclonal antibodies have great potential for treating rarer diseases, as well as conveying passive immunity to patients (as was done with some Covid-19 therapies).