Faith is, in general terms, the act of believing in something that you can't prove. This does not necessarily make it a less valid belief system than something you have proven. There are many things that can't be or haven't been proven, but can't be ignored. Your use of faith will probably be on the same level as your use of reason: if you have a well-developed understanding of the world, your faith will not need to fill in gaps that it should not; if you have a well-developed spirituality, your rationality will not have to fill in gaps that it should not.

Faith isn't only for religion. You also probably have faith in our government, some other governmental system, or anarchy. Most of these haven't been tested out thoroughly, and none of them have been tested under all of the conditions that will come along in the next ten years (and with any luck, you'll still be around in 30 years -- scary, huh?)

Most people have faith in moral systems that are unprovable; it is arguable whether it is possible to 'prove' that a moral system is the best, and it is certain that most people simply find one that is serviceable and put their faith in it.

On a deeper level, philosophers have long wondered how we know that the world we see is real in the sense that most people think it is. The most famous example of this is Descartes' Method of Doubt ("I think, therefore I am") or, perhaps, The Matrix's mind control. We have no reason to think that the 'real world' is more real than the ones that exist in our mind except for convenience and faith.

Faith is a reasonable response to these situations. If you don't have a logical explanation, it is logical to pick what seems best or what has worked for others and stick to it until you find something more empirical to experiment with. So it is reasonable to view faith is a mechanism allowing humans to cope with a largely mysterious world. Humans may get carried away and have wars over their faith, but this does not mean that faith is bad -- humans will have wars over just about anything. There are, however, a lot of people who take their faith on certain subjects as an absolute, unquestionable given. This can be bad, as faith is a strong tool, and if it is over-applied, it can get in the way of understanding the world.