Sunblock and sunscreen are basically the same thing, but if you want the most effective sun protection, use one containing both chemical AND physical protection.
Sunblock or Sunscreen contains chemicals that chemically reflect the sun's rays, and that's pretty good. They won't stop you from tanning altogether [Meaning, the sun is being filtered, not completely blocked], but Broad Spectrum Sunscreen/block will chemically reflect both UVA and UVB rays, protecting your skin from both short and long UV rays, as Vtmom said above!
Zinc, however, contains particles that actually physically block the suns rays (Both UVA and UVB), giving you the best protection available [Perhaps, apart from shelter and covering up!]. Recently, Megan Gale has Introduced "Invisible Zinc", which actually contains both chemical and physical forms of protection. The particles are tiny, meaning that this stuff is actually completely transparent, unlike the white, chalky substance that it used to be! It comes in transparent, and tinted/moisturising formulas. I absolutely love it! Its quite matte, its transparent, and it gives the best protection.
Apart from this, Dermalogica's sunscreen is also excellent: I like to mix it with my moisturiser!