It is well-known that droplets projected by coughing or sneezing are an important factor, either directly by inhalation or indirectly through surface contamination (in particular in the case of metallic or made of plastic surfaces), then transfer by hand-face contact (nose, eyes, mouth) hence the importance of washing hands regularly. Using a mask protects by partially blocking the inhalation of particles and inhibiting hand-mouth and hand-nose contact (wearing glasses also helps), and reversely by blocking the emission of contaminated droplets by the wearer of the mask.
Another factor of transmission is being studied by researchers in Japan: micro-droplets. Beyond visible droplets mentioned above, measuring circa 1mm in diameter and would fall on the ground and other surfaces after circa 1 minute, coughing, sneezing and even simple conversation generate invisible micro-droplets of circa 10 µm in diameter who can float for more than 20 minutes in environments without proper ventilation.
Even though the influence of micro-droplets in the propagation is not yet well understood, masks clearly can prevent transmission via this channel as, being 50 to 100 times larger than the virus itself, they lay within the range of efficiency of most homemade masks.