What is the difference between a sunscreen and a sun block? Is it possible to burn while using a sunblock?

A chemically-based sunscreen contains one of several ingredients that absorb most of the ultraviolet rays, prolonging the time in which skin can be exposed to sunlight without burning. A sunblock is a physical sunscreen, i.e., a barrier against the ultraviolet rays of the sun. Sunblocks are available in creams or ointments containing titanium dioxide or the familiar white zinc oxide. If applied correctly, completely covering an area, such as the nose or lips, sunblocks are very effective in preventing sunburn. Because of their thick, greasy consistency they are inappropriate for oily skin types.