Basically, the main difference is price and bragging rights. Deore is usually Shimano's entry level components. But it doesn't mean because it's entry level it's no good. XT and XTR is Shimano's high end components. With the XT and XTR components they are usually a few grams lighter, looks a bit better / less plastic, and are supposed to be smoother. I've got a mix of SLX and XT on my bikes and I can't tell which are smoother, lighter etc, but I know the XT cost a lot more. SLX is a safe bet and in many instances SLX is actually as good as XT or even better. For instance, the SLX crankset is highly rates, as are the brakes and gear components. I would personally not pay more for XT over SLX - if it comes on a new bike I wouldn't say no but I wouldn't pay more for it as there would be no benefit other than "badge appeal".
