An appeal to faith is arguably a logical fallacy in which one claims that evidence is not present and may not even be possible, but that one must have faith and accept an unsupported fact.
This is not clearly a fallacy; if you ask why I believe in God, and I say it is simply because I have faith, this is not inherently irrational. It is not an argument that should be very convincing, and it could go extremely badly (e.g., Jesus, take the wheel is a bad mandate to take literally), but it is not inherently fallacious.
Sadly, it is often used as the motte in a motte and bailey argument, which is clearly an abuse of reason. Arguments of this sort may look something like this:
Tem42: "We should send Bibles to Africa because God is good!"
Reader: "Isn't this the same god that kills 30,000 babies a year?"
Tem42: "We may not always understand God's methods, but I have faith that He has a plan."
This should not be convincing, as my faith has no bearing on your opinions or actions (aside from any desire you might have to protect my feelings), because no evidence has been put forth to suggest that God is good, and because God has not told you to send Bibles to Africa.
The final statement however, is technically unassailable. Bereft of any evidence against my personal experience of faith, and unable to convince someone who does not put evidence above happy dreams, you have only two choices. You can either nod, smile, and walk away, or explain that my faith is bad and I should feel bad. Most people will opt for the former.
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