Low resistance if you have an eGo or Riva 510 type "fat batt" would be a 2 ohm or lower Boge cartomizer (carto) or atomizer (atty); I found 2 ohms to be pleasant, though some people like to use lower resistance.
If you have a KR808D-1 (808) type kit, low resistance would be around a 2.5 ohm cartomizer or atomizer. The Volt and Bloog Max Fusion (BMF) are representatives of a new generation 808, I'm not sure how much better/different they are (I still fall back to a normal 808 battery if I need something smaller and automatic).
It's about power -- a bit more power (measured in Watts) provides greater warmth/vapor/throat hit, though if you go too high you more or less lose flavor, "burn" the eliquid. For a given battery voltage, lower resistance increases the power. You generally want to avoid going too low on resistance, not just due to too much heat but because too high a current draw can lead to early battery or electronics failure (personal vaporizers, PVs, typically contain some electronics to regulate the output voltage, which can only handle so many amps of current; too much current and they fail early).
I've used up to 100%VG (nominal) with cartos without issue. The trick is that this is more accurately described as 0PG than 100%VG -- a bit of pure grain alcohol (PGA) (or high proof unflavored vodka) or distilled water is used to thin the liquid. When people say that so-and-so's 100% VG eliquid is pretty thin, wicks well, that vendor actually has to be using water, or PGA, or another vapable base to thin it; VG by itself is inherently thick, that's a chemical property.
