What's the difference between sunblock or sunscreen?

Many: Sunblocks contain ingredients that reflect the uv rays to prevent damage whereas sunscreens contain chemicals which absorb the uv energy, thus lessening damage. Typically blocks are opaque and screens transparent but that is not always true and the differences are further complicated because manufacturers often mix chemicals/mislabel. Blocks tend to protect more and be less irritating.

Shield vs filter: True sunblocks essentially create a shield to effectively block out the sun with zinc oxide being one of the most common preparations. They tend to be opaque. Sunscreens are chemical agents which filter out undersireable wavelengths of the sun and come in a variety of strengths and formulations. They tend to be clear. The diagram represents the spectrum of coverage.