Penetrating power refers to how deeply a type of radiation can travel into a material before being absorbed. Alpha particles are heavy, positively charged helium nuclei. Their large mass and charge cause them to interact strongly with matter, losing energy very quickly. Therefore, they are stopped by just a few centimeters of air or a sheet of paper.

In comparison, $\beta$-particles (electrons) are lighter and smaller, and gamma rays are massless, high-energy photons. They can penetrate much farther through materials like aluminum or lead, respectively.

Final Answer: Alpha particles