250K = warmer
The usual choice for single coil pickups.
500K = brighter
The usual choice for humbuckers.
1Meg-ohm = brightest
If you want to hear your guitar "wide open" with all the highs, try 1 Meg pots.
1-meg pots are also ideal for piezo pickups, as the ultra high impedance piezo element is more adversely effected by conventional, lower-value pots than magnetic pickups.
are used with active systems with battery-powered preamps.
These "K" numbers are the resistance ratings (kOhms). K is the abbreviation for Kilo, or 1,000. 1Meg-ohm is 1,000K, or 1-million ohms.
You can experiment with different pots on your guitar: higher values produce a slightly brighter tone. Lower values produce a slightly warmer tone (they remove little more of the highs).