Spark Plug Corrosion / Residue?

[BuzzKill]

I replaced these plugs about 1 year/10k miles ago.  The bottom plug in the picture is from the cylinder 6 (rear left), and the top is from 5 (opposite).  All the plugs on the left have about the same look as #6 (thick white residue and what looks like rust on outside) and all the ones on the right look like #5 (mostly clean with slight white residue on outside). 

My questions are,
1.  Is it effecting preformance, such as mileage?
2.  What is causing it?
3.  Do I need to replace em?
4.  How can I keep it from happening again. 

[Skinhead]

Looks like water is pooling in the spark plug hole of the head.  I believe there is a drain hole in each plug well, I would check and make sure they are open and draining.  If the plug well is filling with water, you may short out the plug when water is present and have a miss/ dead cylinder until the water evaporates.

[Michvalk]

Looks like water is pooling in the spark plug hole of the head.  I believe there is a drain hole in each plug well, I would check and make sure they are open and draining.  If the plug well is filling with water, you may short out the plug when water is present and have a miss/ dead cylinder until the water evaporates.

+1. And,yes, I would replace the plugs

[98valk]

+1 what skinhead said.  wire brush them and use anti-sieze on threads.

[Thunderbolt]

in the spark plug well.

[Robert]

You guys have it handled  except for the color on the center electrode looks really good