I have a two part question:

1. Does paying off your credit cards in full each month have a negavite/positive/no effect on your credit? I ask this because I would assume credit card companies would prefer to make money off of interest, but I guess there is a risk in taking someone who builds a big balance.

2. Can someone explain a balance transfer for me? I recently got a new card and was considering doing this. It is 0% APR, but I noticed that there is a fee associated with transfers. I forgot the exact number, but it is a percentage with a maximum dollar amount. Does anyone know if this normally applies during the 0% APR period also?
In addition to number one...

So does carrying a balance help your credit score? I'd prefer not to pay interest if it isn't necessary.